HTML Standard

This specification defines a big part of the Web platform, in lots of detail. Its place in the Web platform specification stack relative to other specifications can be best summed up as follows:

1.2 Is this HTML5?

In more length: the term "HTML5" is widely used as a buzzword to refer to modern Web technologies, many of which (though by no means all) are developed at the WHATWG. This document is one such; others are available from the WHATWG specification index.

Although we have asked them to stop doing so, the W3C also republishes some parts of this specification as separate documents.

1.3 Background

HTML is the World Wide Web's core markup language. Originally, HTML was primarily designed as a language for semantically describing scientific documents. Its general design, however, has enabled it to be adapted, over the subsequent years, to describe a number of other types of documents and even applications.

1.4 Audience

This specification is intended for authors of documents and scripts that use the features defined in this specification, implementers of tools that operate on pages that use the features defined in this specification, and individuals wishing to establish the correctness of documents or implementations with respect to the requirements of this specification.

This document is probably not suited to readers who do not already have at least a passing familiarity with Web technologies, as in places it sacrifices clarity for precision, and brevity for completeness. More approachable tutorials and authoring guides can provide a gentler introduction to the topic.

In particular, familiarity with the basics of DOM is necessary for a complete understanding of some of the more technical parts of this specification. An understanding of Web IDL, HTTP, XML, Unicode, character encodings, JavaScript, and CSS will also be helpful in places but is not essential.

1.5 Scope

This specification is limited to providing a semantic-level markup language and associated semantic-level scripting APIs for authoring accessible pages on the Web ranging from static documents to dynamic applications.

The scope of this specification does not include providing mechanisms for media-specific customization of presentation (although default rendering rules for Web browsers are included at the end of this specification, and several mechanisms for hooking into CSS are provided as part of the language).

The scope of this specification is not to describe an entire operating system. In particular, hardware configuration software, image manipulation tools, and applications that users would be expected to use with high-end workstations on a daily basis are out of scope. In terms of applications, this specification is targeted specifically at applications that would be expected to be used by users on an occasional basis, or regularly but from disparate locations, with low CPU requirements. Examples of such applications include online purchasing systems, searching systems, games (especially multiplayer online games), public telephone books or address books, communications software (e-mail clients, instant messaging clients, discussion software), document editing software, etc.

1.6 History

For its first five years (1990-1995), HTML went through a number of revisions and experienced a number of extensions, primarily hosted first at CERN, and then at the IETF.

With the creation of the W3C, HTML's development changed venue again. A first abortive attempt at extending HTML in 1995 known as HTML 3.0 then made way to a more pragmatic approach known as HTML 3.2, which was completed in 1997. HTML4 quickly followed later that same year.

The following year, the W3C membership decided to stop evolving HTML and instead begin work on an XML-based equivalent, called XHTML. This effort started with a reformulation of HTML4 in XML, known as XHTML 1.0, which added no new features except the new serialization, and which was completed in 2000. After XHTML 1.0, the W3C's focus turned to making it easier for other working groups to extend XHTML, under the banner of XHTML Modularization. In parallel with this, the W3C also worked on a new language that was not compatible with the earlier HTML and XHTML languages, calling it XHTML2.

Around the time that HTML's evolution was stopped in 1998, parts of the API for HTML developed by browser vendors were specified and published under the name DOM Level 1 (in 1998) and DOM Level 2 Core and DOM Level 2 HTML (starting in 2000 and culminating in 2003). These efforts then petered out, with some DOM Level 3 specifications published in 2004 but the working group being closed before all the Level 3 drafts were completed.

In 2003, the publication of XForms, a technology which was positioned as the next generation of Web forms, sparked a renewed interest in evolving HTML itself, rather than finding replacements for it. This interest was borne from the realization that XML's deployment as a Web technology was limited to entirely new technologies (like RSS and later Atom), rather than as a replacement for existing deployed technologies (like HTML).

A proof of concept to show that it was possible to extend HTML4's forms to provide many of the features that XForms 1.0 introduced, without requiring browsers to implement rendering engines that were incompatible with existing HTML Web pages, was the first result of this renewed interest. At this early stage, while the draft was already publicly available, and input was already being solicited from all sources, the specification was only under Opera Software's copyright.

The idea that HTML's evolution should be reopened was tested at a W3C workshop in 2004, where some of the principles that underlie the HTML5 work (described below), as well as the aforementioned early draft proposal covering just forms-related features, were presented to the W3C jointly by Mozilla and Opera. The proposal was rejected on the grounds that the proposal conflicted with the previously chosen direction for the Web's evolution; the W3C staff and membership voted to continue developing XML-based replacements instead.

Shortly thereafter, Apple, Mozilla, and Opera jointly announced their intent to continue working on the effort under the umbrella of a new venue called the WHATWG. A public mailing list was created, and the draft was moved to the WHATWG site. The copyright was subsequently amended to be jointly owned by all three vendors, and to allow reuse of the specification.

The WHATWG was based on several core principles, in particular that technologies need to be backwards compatible, that specifications and implementations need to match even if this means changing the specification rather than the implementations, and that specifications need to be detailed enough that implementations can achieve complete interoperability without reverse-engineering each other.

The latter requirement in particular required that the scope of the HTML5 specification include what had previously been specified in three separate documents: HTML4, XHTML1, and DOM2 HTML. It also meant including significantly more detail than had previously been considered the norm.

In 2006, the W3C indicated an interest to participate in the development of HTML5 after all, and in 2007 formed a working group chartered to work with the WHATWG on the development of the HTML5 specification. Apple, Mozilla, and Opera allowed the W3C to publish the specification under the W3C copyright, while keeping a version with the less restrictive license on the WHATWG site.

For a number of years, both groups then worked together. In 2011, however, the groups came to the conclusion that they had different goals: the W3C wanted to publish a "finished" version of "HTML5", while the WHATWG wanted to continue working on a Living Standard for HTML, continuously maintaining the specification rather than freezing it in a state with known problems, and adding new features as needed to evolve the platform.

Since then, the WHATWG has been working on this specification (amongst others), and the W3C has been copying fixes made by the WHATWG into their fork of the document (which also has other changes).

1.7 Design notes

It must be admitted that many aspects of HTML appear at first glance to be nonsensical and inconsistent.

HTML, its supporting DOM APIs, as well as many of its supporting technologies, have been developed over a period of several decades by a wide array of people with different priorities who, in many cases, did not know of each other's existence.

Features have thus arisen from many sources, and have not always been designed in especially consistent ways. Furthermore, because of the unique characteristics of the Web, implementation bugs have often become de-facto, and now de-jure, standards, as content is often unintentionally written in ways that rely on them before they can be fixed.

Despite all this, efforts have been made to adhere to certain design goals. These are described in the next few subsections.

1.7.1 Serializability of script execution

To avoid exposing Web authors to the complexities of multithreading, the HTML and DOM APIs are designed such that no script can ever detect the simultaneous execution of other scripts. Even with workers, the intent is that the behavior of implementations can be thought of as completely serializing the execution of all scripts in all browsing contexts.

The exception to this general design principle is the JavaScript SharedArrayBuffer class. Using SharedArrayBuffer objects, it can in fact be observed that scripts in other agents are executing simultaneously. Furthermore, due to the JavaScript memory model, there are situations which not only are un-representable via serialized script execution, but also un-representable via serialized statement execution among those scripts.

1.7.2 Compliance with other specifications

This specification interacts with and relies on a wide variety of other specifications. In certain circumstances, unfortunately, conflicting needs have led to this specification violating the requirements of these other specifications. Whenever this has occurred, the transgressions have each been noted as a "willful violation", and the reason for the violation has been noted.

1.7.3 Extensibility

HTML has a wide array of extensibility mechanisms that can be used for adding semantics in a safe manner:

1.8 HTML vs XML syntax

This specification defines an abstract language for describing documents and applications, and some APIs for interacting with in-memory representations of resources that use this language.

There are various concrete syntaxes that can be used to transmit resources that use this abstract language, two of which are defined in this specification.

The first such concrete syntax is the HTML syntax. This is the format suggested for most authors. It is compatible with most legacy Web browsers. If a document is transmitted with the text/html MIME type, then it will be processed as an HTML document by Web browsers. This specification defines the latest HTML syntax, known simply as "HTML".

The second concrete syntax is XML. When a document is transmitted with an XML MIME type, such as application/xhtml+xml, then it is treated as an XML document by Web browsers, to be parsed by an XML processor. Authors are reminded that the processing for XML and HTML differs; in particular, even minor syntax errors will prevent a document labeled as XML from being rendered fully, whereas they would be ignored in the HTML syntax.

The XML syntax for HTML was formerly referred to as "XHTML", but this specification does not use that term (among other reasons, because no such term is used for the HTML syntaxes of MathML and SVG).

The DOM, the HTML syntax, and the XML syntax cannot all represent the same content. For example, namespaces cannot be represented using the HTML syntax, but they are supported in the DOM and in the XML syntax. Similarly, documents that use the noscript feature can be represented using the HTML syntax, but cannot be represented with the DOM or in the XML syntax. Comments that contain the string "-->" can only be represented in the DOM, not in the HTML and XML syntaxes.

1.9 Structure of this specification

1.9.1 How to read this specification

This specification should be read like all other specifications. First, it should be read cover-to-cover, multiple times. Then, it should be read backwards at least once. Then it should be read by picking random sections from the contents list and following all the cross-references.

As described in the conformance requirements section below, this specification describes conformance criteria for a variety of conformance classes. In particular, there are conformance requirements that apply to producers, for example authors and the documents they create, and there are conformance requirements that apply to consumers, for example Web browsers. They can be distinguished by what they are requiring: a requirement on a producer states what is allowed, while a requirement on a consumer states how software is to act.

1.9.2 Typographic conventions

The defining instance of a term is marked up like this. Uses of that term are marked up like this or like this.

The defining instance of an element, attribute, or API is marked up like this. References to that element, attribute, or API are marked up like this.

In some cases, requirements are given in the form of lists with conditions and corresponding requirements. In such cases, the requirements that apply to a condition are always the first set of requirements that follow the condition, even in the case of there being multiple sets of conditions for those requirements. Such cases are presented as follows:

1.10 Privacy concerns

In general, due to the Internet's architecture, a user can be distinguished from another by the user's IP address. IP addresses do not perfectly match to a user; as a user moves from device to device, or from network to network, their IP address will change; similarly, NAT routing, proxy servers, and shared computers enable packets that appear to all come from a single IP address to actually map to multiple users. Technologies such as onion routing can be used to further anonymize requests so that requests from a single user at one node on the Internet appear to come from many disparate parts of the network.

However, the IP address used for a user's requests is not the only mechanism by which a user's requests could be related to each other. Cookies, for example, are designed specifically to enable this, and are the basis of most of the Web's session features that enable you to log into a site with which you have an account.

There are other mechanisms that are more subtle. Certain characteristics of a user's system can be used to distinguish groups of users from each other; by collecting enough such information, an individual user's browser's "digital fingerprint" can be computed, which can be as good as, if not better than, an IP address in ascertaining which requests are from the same user.

Grouping requests in this manner, especially across multiple sites, can be used for both benign (and even arguably positive) purposes, as well as for malevolent purposes. An example of a reasonably benign purpose would be determining whether a particular person seems to prefer sites with dog illustrations as opposed to sites with cat illustrations (based on how often they visit the sites in question) and then automatically using the preferred illustrations on subsequent visits to participating sites. Malevolent purposes, however, could include governments combining information such as the person's home address (determined from the addresses they use when getting driving directions on one site) with their apparent political affiliations (determined by examining the forum sites that they participate in) to determine whether the person should be prevented from voting in an election.

Since the malevolent purposes can be remarkably evil, user agent implementers are encouraged to consider how to provide their users with tools to minimize leaking information that could be used to fingerprint a user.

Unfortunately, as the first paragraph in this section implies, sometimes there is great benefit to be derived from exposing the very information that can also be used for fingerprinting purposes, so it's not as easy as simply blocking all possible leaks. For instance, the ability to log into a site to post under a specific identity requires that the user's requests be identifiable as all being from the same user, more or less by definition. More subtly, though, information such as how wide text is, which is necessary for many effects that involve drawing text onto a canvas (e.g. any effect that involves drawing a border around the text) also leaks information that can be used to group a user's requests. (In this case, by potentially exposing, via a brute force search, which fonts a user has installed, information which can vary considerably from user to user.)

Features in this specification which can be used to fingerprint the user are marked as this paragraph is.

Other features in the platform can be used for the same purpose, though, including, though not limited to:

1.10.1 Cross-site communication

The postMessage() API provides a mechanism by which two sites can communicate directly. At first glance, this might appear to open a new way by which the problems described above can occur. However, in practice, multiple mechanisms exist by which two sites can communicate that predate this API: a site embedding another can send data via an iframe element's dimensions; a site can use a cross-site image request with a unique identifier known to the server to initiate a server-side data exchange; or indeed the fingerprinting techniques described above can be used by two sites to uniquely identify a visitor such that information can then be exchanged on the server side.

Fundamentally, users that do not trust a site to treat their information with respect have to avoid visiting that site at all.

1.11 A quick introduction to HTML

HTML documents consist of a tree of elements and text. Each element is denoted in the source by a start tag, such as "<body>", and an end tag, such as "</body>". (Certain start tags and end tags can in certain cases be omitted and are implied by other tags.)

Tags have to be nested such that elements are all completely within each other, without overlapping:

This specification defines a set of elements that can be used in HTML, along with rules about the ways in which the elements can be nested.

Elements can have attributes, which control how the elements work. In the example below, there is a hyperlink, formed using the a element and its href attribute:

Attributes are placed inside the start tag, and consist of a name and a value, separated by an "=" character. The attribute value can remain unquoted if it doesn't contain ASCII whitespace or any of " ' ` = < or >. Otherwise, it has to be quoted using either single or double quotes. The value, along with the "=" character, can be omitted altogether if the value is the empty string.

HTML user agents (e.g. Web browsers) then parse this markup, turning it into a DOM (Document Object Model) tree. A DOM tree is an in-memory representation of a document.

The document element of this tree is the html element, which is the element always found in that position in HTML documents. It contains two elements, head and body, as well as a Text node between them.

There are many more Text nodes in the DOM tree than one would initially expect, because the source contains a number of spaces (represented here by "␣") and line breaks ("⏎") that all end up as Text nodes in the DOM. However, for historical reasons not all of the spaces and line breaks in the original markup appear in the DOM. In particular, all the whitespace before head start tag ends up being dropped silently, and all the whitespace after the body end tag ends up placed at the end of the body.

The head element contains a title element, which itself contains a Text node with the text "Sample page". Similarly, the body element contains an h1 element, a p element, and a comment.

This DOM tree can be manipulated from scripts in the page. Scripts (typically in JavaScript) are small programs that can be embedded using the script element or using event handler content attributes. For example, here is a form with a script that sets the value of the form's output element to say "Hello World":

Each element in the DOM tree is represented by an object, and these objects have APIs so that they can be manipulated. For instance, a link (e.g. the a element in the tree above) can have its "href" attribute changed in several ways:

Since DOM trees are used as the way to represent HTML documents when they are processed and presented by implementations (especially interactive implementations like Web browsers), this specification is mostly phrased in terms of DOM trees, instead of the markup described above.

HTML documents represent a media-independent description of interactive content. HTML documents might be rendered to a screen, or through a speech synthesizer, or on a braille display. To influence exactly how such rendering takes place, authors can use a styling language such as CSS.

For more details on how to use HTML, authors are encouraged to consult tutorials and guides. Some of the examples included in this specification might also be of use, but the novice author is cautioned that this specification, by necessity, defines the language with a level of detail that might be difficult to understand at first.

1.11.1 Writing secure applications with HTML

When HTML is used to create interactive sites, care needs to be taken to avoid introducing vulnerabilities through which attackers can compromise the integrity of the site itself or of the site's users.

A comprehensive study of this matter is beyond the scope of this document, and authors are strongly encouraged to study the matter in more detail. However, this section attempts to provide a quick introduction to some common pitfalls in HTML application development.

The security model of the Web is based on the concept of "origins", and correspondingly many of the potential attacks on the Web involve cross-origin actions. [ORIGIN]

1.11.2 Common pitfalls to avoid when using the scripting APIs

Scripts in HTML have "run-to-completion" semantics, meaning that the browser will generally run the script uninterrupted before doing anything else, such as firing further events or continuing to parse the document.

On the other hand, parsing of HTML files happens incrementally, meaning that the parser can pause at any point to let scripts run. This is generally a good thing, but it does mean that authors need to be careful to avoid hooking event handlers after the events could have possibly fired.

There are two techniques for doing this reliably: use event handler content attributes, or create the element and add the event handlers in the same script. The latter is safe because, as mentioned earlier, scripts are run to completion before further events can fire.

1.11.3 How to catch mistakes when writing HTML: validators and conformance checkers

Authors are encouraged to make use of conformance checkers (also known as validators) to catch common mistakes. The WHATWG maintains a list of such tools at: https://whatwg.org/validator/

1.12 Conformance requirements for authors

Unlike previous versions of the HTML specification, this specification defines in some detail the required processing for invalid documents as well as valid documents.

However, even though the processing of invalid content is in most cases well-defined, conformance requirements for documents are still important: in practice, interoperability (the situation in which all implementations process particular content in a reliable and identical or equivalent way) is not the only goal of document conformance requirements. This section details some of the more common reasons for still distinguishing between a conforming document and one with errors.

1.12.1 Presentational markup

The majority of presentational features from previous versions of HTML are no longer allowed. Presentational markup in general has been found to have a number of problems:

For those reasons, presentational markup has been removed from HTML in this version. This change should not come as a surprise; HTML4 deprecated presentational markup many years ago and provided a mode (HTML4 Transitional) to help authors move away from presentational markup; later, XHTML 1.1 went further and obsoleted those features altogether.

The only remaining presentational markup features in HTML are the style attribute and the style element. Use of the style attribute is somewhat discouraged in production environments, but it can be useful for rapid prototyping (where its rules can be directly moved into a separate style sheet later) and for providing specific styles in unusual cases where a separate style sheet would be inconvenient. Similarly, the style element can be useful in syndication or for page-specific styles, but in general an external style sheet is likely to be more convenient when the styles apply to multiple pages.

It is also worth noting that some elements that were previously presentational have been redefined in this specification to be media-independent: b, i, hr, s, small, and u.

1.12.2 Syntax errors

Some authors find it helpful to be in the practice of always quoting all attributes and always including all optional tags, preferring the consistency derived from such custom over the minor benefits of terseness afforded by making use of the flexibility of the HTML syntax. To aid such authors, conformance checkers can provide modes of operation wherein such conventions are enforced.

1.12.3 Restrictions on content models and on attribute values

Beyond the syntax of the language, this specification also places restrictions on how elements and attributes can be specified. These restrictions are present for similar reasons:

1.13 Suggested reading

2 Common infrastructure

2.1 Terminology

This specification refers to both HTML and XML attributes and IDL attributes, often in the same context. When it is not clear which is being referred to, they are referred to as content attributes for HTML and XML attributes, and IDL attributes for those defined on IDL interfaces. Similarly, the term "properties" is used for both JavaScript object properties and CSS properties. When these are ambiguous they are qualified as object properties and CSS properties respectively.

Generally, when the specification states that a feature applies to the HTML syntax or the XML syntax, it also includes the other. When a feature specifically only applies to one of the two languages, it is called out by explicitly stating that it does not apply to the other format, as in "for HTML, ... (this does not apply to XML)".

This specification uses the term document to refer to any use of HTML, ranging from short static documents to long essays or reports with rich multimedia, as well as to fully-fledged interactive applications. The term is used to refer both to Document objects and their descendant DOM trees, and to serialized byte streams using the HTML syntax or the XML syntax, depending on context.

In the context of the DOM structures, the terms HTML document and XML document are used as defined in DOM, and refer specifically to two different modes that Document objects can find themselves in. [DOM] (Such uses are always hyperlinked to their definition.)

In the context of byte streams, the term HTML document refers to resources labeled as text/html, and the term XML document refers to resources labeled with an XML MIME type.

For simplicity, terms such as shown, displayed, and visible might sometimes be used when referring to the way a document is rendered to the user. These terms are not meant to imply a visual medium; they must be considered to apply to other media in equivalent ways.

2.1.1 Parallelism

To run steps in parallel means those steps are to be run, one after another, at the same time as other logic in the standard (e.g., at the same time as the event loop). This standard does not define the precise mechanism by which this is achieved, be it time-sharing cooperative multitasking, fibers, threads, processes, using different hyperthreads, cores, CPUs, machines, etc. By contrast, an operation that is to run immediately must interrupt the currently running task, run itself, and then resume the previously running task.

To avoid race conditions between different in parallel algorithms that operate on the same data, a parallel queue can be used.

A parallel queue represents a queue of algorithm steps that must be run in series.

Steps running in parallel can themselves run other steps in in parallel. E.g., inside a parallel queue it can be useful to run a series of steps in parallel with the queue.

2.1.2 Resources

The specification uses the term supported when referring to whether a user agent has an implementation capable of decoding the semantics of an external resource. A format or type is said to be supported if the implementation can process an external resource of that format or type without critical aspects of the resource being ignored. Whether a specific resource is supported can depend on what features of the resource's format are in use.

For example, a PNG image would be considered to be in a supported format if its pixel data could be decoded and rendered, even if, unbeknownst to the implementation, the image also contained animation data.

An MPEG-4 video file would not be considered to be in a supported format if the compression format used was not supported, even if the implementation could determine the dimensions of the movie from the file's metadata.

What some specifications, in particular the HTTP specifications, refer to as a representation is referred to in this specification as a resource. [HTTP]

A resource's critical subresources are those that the resource needs to have available to be correctly processed. Which resources are considered critical or not is defined by the specification that defines the resource's format.

For CSS style sheets, we tentatively define here that their critical subresources are other style sheets imported via @import rules, including those indirectly imported by other imported style sheets.

This definition is not fully interoperable; furthermore, some user agents seem to count resources like background images or web fonts as critical subresources. Ideally, the CSS Working Group would define this; see w3c/csswg-drafts issue #1088 to track progress on that front.

2.1.3 XML compatibility

To ease migration from HTML to XML, UAs conforming to this specification will place elements in HTML in the http://www.w3.org/1999/xhtml namespace, at least for the purposes of the DOM and CSS. The term "HTML elements" refers to any element in that namespace, even in XML documents.

Except where otherwise stated, all elements defined or mentioned in this specification are in the HTML namespace ("http://www.w3.org/1999/xhtml"), and all attributes defined or mentioned in this specification have no namespace.

The term element type is used to refer to the set of elements that have a given local name and namespace. For example, button elements are elements with the element type button, meaning they have the local name "button" and (implicitly as defined above) the HTML namespace.

Attribute names are said to be XML-compatible if they match the Name production defined in XML and they contain no U+003A COLON characters (:). [XML]

2.1.4 DOM trees

When it is stated that some element or attribute is ignored, or treated as some other value, or handled as if it was something else, this refers only to the processing of the node after it is in the DOM. A user agent must not mutate the DOM in such situations.

A content attribute is said to change value only if its new value is different than its previous value; setting an attribute to a value it already has does not change it.

The term empty, when used for an attribute value, Text node, or string, means that the length of the text is zero (i.e., not even containing controls or U+0020 SPACE).

A node A is inserted into a node B when the insertion steps are invoked with A as the argument and A's new parent is B. Similarly, a node A is removed from a node B when the removing steps are invoked with A as the removedNode argument and B as the oldParent argument.

A node is inserted into a document when the insertion steps are invoked with it as the argument and it is now in a document tree. Analogously, a node is removed from a document when the removing steps are invoked with it as the argument and it is now no longer in a document tree.

A node becomes connected when the insertion steps are invoked with it as the argument and it is now connected. Analogously, a node becomes disconnected when the removing steps are invoked with it as the argument and it is now no longer connected.

2.1.5 Scripting

The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo".

An IDL attribute is said to be getting when its value is being retrieved (e.g. by author script), and is said to be setting when a new value is assigned to it.

If a DOM object is said to be live, then the attributes and methods on that object must operate on the actual underlying data, not a snapshot of the data.

2.1.6 Plugins

The term plugin refers to a user-agent defined set of content handlers used by the user agent that can take part in the user agent's rendering of a Document object, but that neither act as child browsing contexts of the Document nor introduce any Node objects to the Document's DOM.

Typically such content handlers are provided by third parties, though a user agent can also designate built-in content handlers as plugins.

One example of a plugin would be a PDF viewer that is instantiated in a browsing context when the user navigates to a PDF file. This would count as a plugin regardless of whether the party that implemented the PDF viewer component was the same as that which implemented the user agent itself. However, a PDF viewer application that launches separate from the user agent (as opposed to using the same interface) is not a plugin by this definition.

This specification does not define a mechanism for interacting with plugins, as it is expected to be user-agent- and platform-specific. Some UAs might opt to support a plugin mechanism such as the Netscape Plugin API; others might use remote content converters or have built-in support for certain types. Indeed, this specification doesn't require user agents to support plugins at all. [NPAPI]

For example, a secured plugin would prevent its contents from creating pop-up windows when the plugin is instantiated inside a sandboxed iframe.

Browsers should take extreme care when interacting with external content intended for plugins. When third-party software is run with the same privileges as the user agent itself, vulnerabilities in the third-party software become as dangerous as those in the user agent.

Since different users having different sets of plugins provides a fingerprinting vector that increases the chances of users being uniquely identified, user agents are encouraged to support the exact same set of plugins for each user.

2.1.7 Character encodings

A character encoding, or just encoding where that is not ambiguous, is a defined way to convert between byte streams and Unicode strings, as defined in Encoding. An encoding has an encoding name and one or more encoding labels, referred to as the encoding's name and labels in the Encoding standard. [ENCODING]

Since support for encodings that are not defined in Encoding is prohibited, UTF-16 encodings are the only encodings that this specification needs to treat as not being ASCII-compatible encodings.

2.1.8 Conformance classes

This specification describes the conformance criteria for user agents (relevant to implementers) and documents (relevant to authors and authoring tool implementers).

Conforming documents are those that comply with all the conformance criteria for documents. For readability, some of these conformance requirements are phrased as conformance requirements on authors; such requirements are implicitly requirements on documents: by definition, all documents are assumed to have had an author. (In some cases, that author may itself be a user agent — such user agents are subject to additional rules, as explained below.)

For example, if a requirement states that "authors must not use the foobar element", it would imply that documents are not allowed to contain elements named foobar.

There is no implied relationship between document conformance requirements and implementation conformance requirements. User agents are not free to handle non-conformant documents as they please; the processing model described in this specification applies to implementations regardless of the conformity of the input documents.

User agents fall into several (overlapping) categories with different conformance requirements.

User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.

For compatibility with existing content and prior specifications, this specification describes two authoring formats: one based on XML, and one using a custom format inspired by SGML (referred to as the HTML syntax). Implementations must support at least one of these two formats, although supporting both is encouraged.

Some conformance requirements are phrased as requirements on elements, attributes, methods or objects. Such requirements fall into two categories: those describing content model restrictions, and those describing implementation behavior. Those in the former category are requirements on documents and authoring tools. Those in the second category are requirements on user agents. Similarly, some conformance requirements are phrased as requirements on authors; such requirements are to be interpreted as conformance requirements on the documents that authors produce. (In other words, this specification does not distinguish between conformance criteria on authors and conformance criteria on documents.)

2.1.9 Dependencies

This specification relies on several other underlying specifications.

Infra

The following terms are defined in Infra: [INFRA]

The general iteration terms while, continue, and break.
code point and its synonym character
surrogate
scalar value
noncharacter
JavaScript string, code unit, and JavaScript string length
scalar value string
string length
ASCII whitespace
control
ASCII digit
ASCII upper hex digit
ASCII lower hex digit
ASCII hex digit
ASCII upper alpha
ASCII lower alpha
ASCII alpha
ASCII alphanumeric
isomorphic decode
ASCII lowercase
ASCII uppercase
ASCII case-insensitive
strip newlines
normalize newlines
strip leading and trailing ASCII whitespace
strip and collapse ASCII whitespace
split a string on ASCII whitespace
split a string on commas
collect a sequence of code points and its associated position variable
skip ASCII whitespace
The ordered map data structure and the associated definitions for value, entry, exists, getting the value of an entry, setting the value of an entry, removing an entry, getting the keys, and iterate
The list data structure and the associated definitions for append, replace, remove, empty, contains, size, is empty, iterate, and clone
The stack data structure and the associated definitions for push and pop
The queue data structure and the associated definitions for enqueue and dequeue
The ordered set data structure and the associated definition for append
The struct specification type and the associated definition for item
The forgiving-base64 encode and forgiving-base64 decode algorithms
HTML namespace
MathML namespace
SVG namespace
XLink namespace
XML namespace
XMLNS namespace

Unicode and Encoding

The Unicode character set is used to represent textual data, and Encoding defines requirements around character encodings. [UNICODE]

This specification introduces terminology based on the terms defined in those specifications, as described earlier.

The following terms are used as defined in Encoding: [ENCODING]

Getting an encoding
Get an output encoding
The generic decode algorithm which takes a byte stream and an encoding and returns a character stream
The UTF-8 decode algorithm which takes a byte stream and returns a character stream, additionally stripping one leading UTF-8 Byte Order Mark (BOM), if any
The UTF-8 decode without BOM algorithm which is identical to UTF-8 decode except that it does not strip one leading UTF-8 Byte Order Mark (BOM)
The encode algorithm which takes a character stream and an encoding and returns a byte stream
The UTF-8 encode algorithm which takes a character stream and returns a byte stream

XML and related specifications

Implementations that support the XML syntax for HTML must support some version of XML, as well as its corresponding namespaces specification, because that syntax uses an XML serialization with namespaces. [XML] [XMLNS]

Data mining tools and other user agents that perform operations on content without running scripts, evaluating CSS or XPath expressions, or otherwise exposing the resulting DOM to arbitrary content, may "support namespaces" by just asserting that their DOM node analogues are in certain namespaces, without actually exposing the namespace strings.

In the HTML syntax, namespace prefixes and namespace declarations do not have the same effect as in XML. For instance, the colon has no special meaning in HTML element names.

The attribute with the tag name xml:space in the XML namespace is defined by Extensible Markup Language (XML). [XML]

The Name production is defined in XML. [XML]

This specification also references the <?xml-stylesheet?> processing instruction, defined in Associating Style Sheets with XML documents. [XMLSSPI]

This specification also non-normatively mentions the XSLTProcessor interface and its transformToFragment() and transformToDocument() methods. [XSLTP]

URLs

The following terms are defined in URL: [URL]

host
public suffix
same site
domain
IPv4 address
IPv6 address
URL
Origin of URLs
Absolute URL
Relative URL
The URL parser and basic URL parser as well as these parser states:
- scheme start state
- host state
- hostname state
- port state
- path start state
- query state
- fragment state
URL record, as well as its individual components:
- scheme
- username
- password
- host
- port
- path
- query
- fragment
- cannot-be-a-base-URL flag
- object
valid URL string
The cannot have a username/password/port concept
The URL serializer
The host parser
The host serializer
Host equals
URL equals
serialize an integer
Default encode set
UTF-8 percent encode
String percent decode
set the username
set the password
The application/x-www-form-urlencoded format
The application/x-www-form-urlencoded serializer

A number of schemes and protocols are referenced by this specification also:

The about: scheme [ABOUT]
The blob: scheme [FILEAPI]
The data: scheme [RFC2397]
The http: scheme [HTTP]
The https: scheme [HTTP]
The mailto: scheme [MAILTO]
The sms: scheme [SMS]
The urn: scheme [URN]

Media fragment syntax is defined in Media Fragments URI. [MEDIAFRAG]

HTTP and related specifications

The following terms are defined in the HTTP specifications: [HTTP]

`Accept` header
`Accept-Language` header
`Cache-Control` header
`Content-Disposition` header
`Content-Language` header
`Last-Modified` header
`Referer` header

The following terms are defined in HTTP State Management Mechanism: [COOKIES]

cookie-string
receives a set-cookie-string
`Cookie` header

The following term is defined in Web Linking: [WEBLINK]

`Link` header

The following terms are defined in MIME Sniffing: [MIMESNIFF]

Fetch

The following terms are defined in Fetch: [FETCH]

about:blank
An HTTP(S) scheme
A network scheme
A fetch scheme
HTTPS state value
CORS protocol
default `User-Agent` value
extract a MIME type
fetch
HTTP-redirect fetch
ok status
navigation request
network error
`Origin` header
process response
set
terminate
the RequestCredentials enumeration
the RequestDestination enumeration
the fetch() method
response and its associated:
- type
- url
- url list
- status
- header list
- body
- internal response
- CSP list
- HTTPS state
- location URL
request and its associated:
- url
- method
- header list
- body
- client
- URL list
- current URL
- reserved client
- replaces client id
- initiator
- destination
- potential destination
- translating a potential destination
- script-like destinations
- priority
- origin
- referrer
- synchronous flag
- mode
- credentials mode
- use-URL-credentials flag
- unsafe-request flag
- cache mode
- redirect mode
- referrer policy
- cryptographic nonce metadata
- integrity metadata
- parser metadata
- reload-navigation flag
- history-navigation flag

The following terms are defined in Referrer Policy: [REFERRERPOLICY]

referrer policy
The `Referrer-Policy` HTTP header
The parse a referrer policy from a `Referrer-Policy` header algorithm
The "no-referrer", "no-referrer-when-downgrade", "origin-when-cross-origin", and "unsafe-url" referrer policies

The following terms are defined in Mixed Content: [MIX]

a priori authenticated URL

Paint Timing

The following terms are defined in Paint Timing: [PAINTTIMING]

mark paint timing

Long Tasks

The following terms are defined in Long Tasks: [LONGTASKS]

report long tasks

Web IDL

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in Web IDL. [WEBIDL]

The following terms are defined in Web IDL:

The Web IDL also defines the following types that are used in Web IDL fragments in this specification:

The term throw in this specification is used as defined in Web IDL. The DOMException type and the following exception names are defined by Web IDL and used by this specification:

When this specification requires a user agent to create a Date object representing a particular time (which could be the special value Not-a-Number), the milliseconds component of that time, if any, must be truncated to an integer, and the time value of the newly created Date object must represent the resulting truncated time.

For instance, given the time 23045 millionths of a second after 01:00 UTC on January 1st 2000, i.e. the time 2000-01-01T00:00:00.023045Z, then the Date object created representing that time would represent the same time as that created representing the time 2000-01-01T00:00:00.023Z, 45 millionths earlier. If the given time is NaN, then the result is a Date object that represents a time value NaN (indicating that the object does not represent a specific instant of time).

JavaScript

Some parts of the language described by this specification only support JavaScript as the underlying scripting language. [JAVASCRIPT]

The term "JavaScript" is used to refer to ECMA-262, rather than the official term ECMAScript, since the term JavaScript is more widely known. Similarly, the MIME type used to refer to JavaScript in this specification is text/javascript, since that is the most commonly used type, despite it being an officially obsoleted type according to RFC 4329. [RFC4329]

The following terms are defined in the JavaScript specification and used in this specification:

active function object
agent and agent cluster
automatic semicolon insertion
The current Realm Record
early error
invariants of the essential internal methods
JavaScript execution context
JavaScript execution context stack
JavaScript realm
running JavaScript execution context
surrounding agent
Well-Known Symbols, including @@hasInstance, @@isConcatSpreadable, @@toPrimitive, and @@toStringTag
Well-Known Intrinsic Objects, including %ArrayPrototype%, %ErrorPrototype%, %EvalErrorPrototype%, %JSONParse%, %ObjProto_valueOf%, %RangeErrorPrototype%, %ReferenceErrorPrototype%, %SyntaxErrorPrototype%, %TypeErrorPrototype%, and %URIErrorPrototype%
The FunctionBody production
The Module production
The Pattern production
The Script production
The Type notation
The Completion Record specification type
The List and Record specification types
The Property Descriptor specification type
The Script Record specification type
The Cyclic Module Record specification type
The Source Text Module Record specification type and its Evaluate and Link methods
The ArrayCreate abstract operation
The Call abstract operation
The Construct abstract operation
The CopyDataBlockBytes abstract operation
The CreateByteDataBlock abstract operation
The CreateDataProperty abstract operation
The DetachArrayBuffer abstract operation
The EnqueueJob abstract operation
The EnumerableOwnProperties abstract operation
The FinishDynamicImport abstract operation
The FunctionCreate abstract operation
The Get abstract operation
The GetActiveScriptOrModule abstract operation
The GetFunctionRealm abstract operation
The HasOwnProperty abstract operation
The HostEnsureCanCompileStrings abstract operation
The HostImportModuleDynamically abstract operation
The HostPromiseRejectionTracker abstract operation
The HostResolveImportedModule abstract operation
The InitializeHostDefinedRealm abstract operation
The IsAccessorDescriptor abstract operation
The IsCallable abstract operation
The IsConstructor abstract operation
The IsDataDescriptor abstract operation
The IsDetachedBuffer abstract operation
The IsSharedArrayBuffer abstract operation
The NewObjectEnvironment abstract operation
The NormalCompletion abstract operation
The OrdinaryGetPrototypeOf abstract operation
The OrdinarySetPrototypeOf abstract operation
The OrdinaryIsExtensible abstract operation
The OrdinaryPreventExtensions abstract operation
The OrdinaryGetOwnProperty abstract operation
The OrdinaryDefineOwnProperty abstract operation
The OrdinaryGet abstract operation
The OrdinarySet abstract operation
The OrdinaryDelete abstract operation
The OrdinaryOwnPropertyKeys abstract operation
The ObjectCreate abstract operation
The ParseModule abstract operation
The ParseScript abstract operation
The RunJobs abstract operation
The SameValue abstract operation
The ScriptEvaluation abstract operation
The SetImmutablePrototype abstract operation
The ToBoolean abstract operation
The ToString abstract operation
The ToUint32 abstract operation
The TypedArrayCreate abstract operation
The Abstract Equality Comparison algorithm
The Strict Equality Comparison algorithm
The Date class
The SharedArrayBuffer class
The TypeError class
The RangeError class
The eval() function
The [[IsHTMLDDA]] internal slot
import()
The typeof operator
The delete operator
The TypedArray Constructors table

User agents that support JavaScript must also implement the import.meta proposal. The following term is defined there, and used in this specification: [JSIMPORTMETA]

import.meta
The HostGetImportMetaProperties abstract operation

Users agents that support JavaScript must also implement ECMAScript Internationalization API. [JSINTL]

User agents that support JavaScript must also implement the BigInt proposal. [JSBIGINT]

DOM

The Document Object Model (DOM) is a representation — a model — of a document and its content. The DOM is not just an API; the conformance criteria of HTML implementations are defined, in this specification, in terms of operations on the DOM. [DOM]

Implementations must support DOM and the events defined in UI Events, because this specification is defined in terms of the DOM, and some of the features are defined as extensions to the DOM interfaces. [DOM] [UIEVENTS]

In particular, the following features are defined in DOM: [DOM]

Attr interface
Comment interface
DOMImplementation interface
Document interface
DocumentOrShadowRoot interface
DocumentFragment interface
DocumentType interface
ChildNode interface
Element interface
attachShadow() method.
An element's shadow root
The retargeting algorithm
Node interface
NodeList interface
ProcessingInstruction interface
ShadowRoot interface
Text interface
node document concept
document type concept
host concept
shadow root concept
HTMLCollection interface
HTMLCollection.length attribute
HTMLCollection.item() method
HTMLCollection.namedItem() method
The terms collection and represented by the collection
DOMTokenList interface
DOMTokenList.value attribute
createDocument() method
createHTMLDocument() method
createElement() method
createElementNS() method
getElementById() method
getElementsByClassName() method
appendChild() method
cloneNode() method
importNode() method
preventDefault() method
id attribute
textContent attribute
The tree, shadow tree, and node tree concepts
The tree order and shadow-including tree order concepts
The child concept
The root and shadow-including root concepts
The inclusive ancestor, shadow-including descendant, shadow-including inclusive descendant, and shadow-including inclusive ancestor concepts
The first child and next sibling concepts
The document element concept
The in a document tree, in a document (legacy), and connected concepts
The slot concept, and its name and assigned nodes
The find flattened slotables algorithm
The assign a slot algorithm
The pre-insert, insert, append, replace, replace all, string replace all, remove, and adopt algorithms for nodes
The insertion steps, removing steps, adopting steps, and child text content change steps hooks for elements
The change, append, remove, replace, and set value algorithms for attributes
The attribute change steps hook for attributes
The attribute list concept
The data of a text node
The child text content of a node
The descendant text content of a node
Event interface
EventTarget interface
The activation behavior hook
The legacy-pre-activation behavior hook
The legacy-canceled-activation behavior hook
The create an event algorithm
The fire an event algorithm
The canceled flag
The dispatch algorithm
EventInit dictionary type
type attribute
target attribute
currentTarget attribute
bubbles attribute
cancelable attribute
composed flag
isTrusted attribute
initEvent() method
add an event listener
addEventListener() method
The remove an event listener and remove all event listeners algorithms
EventListener callback interface
The type of an event
An event listener and its type and callback
The encoding (herein the character encoding) and content type of a Document
The distinction between XML documents and HTML documents
The terms quirks mode, limited-quirks mode, and no-quirks mode
The algorithm to clone a Node, and the concept of cloning steps used by that algorithm
The concept of base URL change steps and the definition of what happens when an element is affected by a base URL change
The concept of an element's unique identifier (ID)
The concept of an element's classes
The term supported tokens
The concept of a DOM range, and the terms start, end, and boundary point as applied to ranges.
The create an element algorithm
The element interface concept
The concepts of custom element state, and of defined and custom elements
An element's custom element definition
An element's is value
MutationObserver interface and mutation observers in general

The following features are defined in UI Events: [UIEVENTS]

The MouseEvent interface
The MouseEvent interface's relatedTarget attribute
MouseEventInit dictionary type
The FocusEvent interface
The FocusEvent interface's relatedTarget attribute
The UIEvent interface
The UIEvent interface's view attribute
auxclick event
click event
dblclick event
mousedown event
mouseenter event
mouseleave event
mousemove event
mouseout event
mouseover event
mouseup event
wheel event
keydown event
keypress event
keyup event

The following features are defined in Touch Events: [TOUCH]

Touch interface
Touch point concept
touchend event

The following features are defined in Pointer Events: [POINTEREVENTS]

pointerup event

This specification sometimes uses the term name to refer to the event's type; as in, "an event named click" or "if the event name is keypress". The terms "name" and "type" for events are synonymous.

The following features are defined in DOM Parsing and Serialization: [DOMPARSING]

The Selection interface is defined in the Selection API specification. [SELECTION]

User agents are encouraged to implement the features described in execCommand. [EXECCOMMAND]

The following parts of Fullscreen API are referenced from this specification, in part to define the rendering of dialog elements, and also to define how the Fullscreen API interacts with HTML: [FULLSCREEN]

top layer (an ordered set) and its add operation
requestFullscreen()
run the fullscreen steps

High Resolution Time provides the current high resolution time and the DOMHighResTimeStamp typedef. [HRT]

File API

This specification uses the following features defined in File API: [FILEAPI]

The Blob interface and its type attribute
The File interface and its name and lastModified attributes
The FileList interface
The concept of a Blob's snapshot state
The concept of read errors
Blob URL Store

Indexed Database API

This specification uses cleanup Indexed Database transactions defined by Indexed Database API. [INDEXEDDB]

Media Source Extensions

The following terms are defined in Media Source Extensions: [MEDIASOURCE]

MediaSource interface
detaching from a media element

Media Capture and Streams

The following terms are defined in Media Capture and Streams: [MEDIASTREAM]

MediaStream interface

XMLHttpRequest

The following features and terms are defined in XMLHttpRequest: [XHR]

XMLHttpRequest interface
XMLHttpRequest.responseXML attribute
ProgressEvent interface
ProgressEvent.lengthComputable attribute
ProgressEvent.loaded attribute
ProgressEvent.total attribute
FormData interface
FormDataEntryValue type
The concept of entry
create an entry

Battery Status

The following features are defined in Battery Status API: [BATTERY]

getBattery() method

Media Queries

Implementations must support Media Queries. The <media-condition> feature is defined therein. [MQ]

CSS modules

While support for CSS as a whole is not required of implementations of this specification (though it is encouraged, at least for Web browsers), some features are defined in terms of specific CSS requirements.

When this specification requires that something be parsed according to a particular CSS grammar, the relevant algorithm in CSS Syntax must be followed, including error handling rules. [CSSSYNTAX]

For example, user agents are required to close all open constructs upon finding the end of a style sheet unexpectedly. Thus, when parsing the string "rgb(0,0,0" (with a missing close-parenthesis) for a color value, the close parenthesis is implied by this error handling rule, and a value is obtained (the color 'black'). However, the similar construct "rgb(0,0," (with both a missing parenthesis and a missing "blue" value) cannot be parsed, as closing the open construct does not result in a viable value.

To parse a CSS <color> value, given a string input with an optional element element, run these steps:

Let color be the result of parsing input as a CSS <color>. [CSSCOLOR]
If color is failure, then return failure.
If color is 'currentcolor', then:
1. If element is not given, then set color to opaque black.
2. Otherwise, set color to the computed value of the 'color' property of element.
Return color.

The following terms and features are defined in Cascading Style Sheets (CSS): [CSS]

viewport
line box
out-of-flow
in-flow
intrinsic dimensions
content area
content box
border box
margin box
border edge
margin edge
collapsing margins
containing block
inline box
block box
The 'margin-top', 'margin-bottom', 'margin-left', and 'margin-right' properties
The 'padding-top', 'padding-bottom', 'padding-left', and 'padding-right' properties
The 'top', 'bottom', 'left', and 'right' properties
The 'float' property
The 'clear' property
The 'width' property
The 'height' property
The 'line-height' property
The 'vertical-align' property
The 'content' property
The 'inline-block' value of the 'display' property
The 'visibility' property

CSS also defines the following border properties: [CSS]

Border properties
	Top	Bottom	Left	Right
Width	'border-top-width'	'border-bottom-width'	'border-left-width'	'border-right-width'
Style	'border-top-style'	'border-bottom-style'	'border-left-style'	'border-right-style'
Color	'border-top-color'	'border-bottom-color'	'border-left-color'	'border-right-color'

The terms intrinsic width and intrinsic height refer to the width dimension and the height dimension, respectively, of intrinsic dimensions.

The basic version of the 'display' property is defined in CSS, and the property is extended by other CSS modules. [CSS] [CSSRUBY] [CSSTABLE]

The following terms and features are defined in CSS Logical Properties: [CSSLOGICAL]

The 'margin-block-start', 'margin-block-end', 'margin-inline-start', and 'margin-inline-end' properties
The 'padding-block-start', 'padding-block-end', 'padding-inline-start', and 'padding-inline-end' properties
The 'border-block-start-width' property
The 'block-size' property
The 'inline-size' property

The following terms and features are defined in CSS Color: [CSSCOLOR]

The term paint source is used as defined in CSS Image Values and Replaced Content to define the interaction of certain HTML elements with the CSS 'element()' function. [CSSIMAGES]

The term default object size and the 'object-fit' property are also defined in CSS Image Values and Replaced Content: [CSSIMAGES]

The following features are defined in CSS Backgrounds and Borders: [CSSBG]

The 'background-color' property
The 'background-image' property
The 'border-radius' property

The following features are defined in CSS Box Alignment: [CSSALIGN]

The 'align-content' property
The 'align-items' property
The 'align-self' property
The 'justify-self' property
The 'justify-content' property
The 'justify-items' property

The following terms and features are defined in CSS Display: [CSSDISPLAY]

The following features are defined in CSS Flexible Box Layout: [CSSFLEXBOX]

The 'flex-direction' property
The 'flex-wrap' property

The following features are defined in CSS Fonts: [CSSFONTS]

The 'font-family' property
The 'font-weight' property
The 'font-size' property
The 'font' property

The following features are defined in CSS Grid Layout: [CSSGRID]

The 'grid-auto-columns' property
The 'grid-auto-flow' property
The 'grid-auto-rows' property
The 'grid-column-gap' property
The 'grid-row-gap' property
The 'grid-template-areas' property
The 'grid-template-columns' property
The 'grid-template-rows' property

The following terms and features are defined in CSS Intrinsic & Extrinsic Sizing: [CSSSIZING]

fit-content inline size

The 'list-style-type' property is defined in CSS Lists and Counters. [CSSLISTS]

The following features are defined in CSS Overflow. [CSSOVERFLOW]

The 'overflow' property and its 'hidden' value
The 'text-overflow' property

The following features are defined in CSS Positioned Layout: [CSSPOSITION]

The 'position' property and its 'static' value

The following features are defined in CSS Multi-column Layout. [CSSMULTICOL]

The 'column-count' property
The 'column-fill' property
The 'column-gap' property
The 'column-rule' property
The 'column-width' property

The 'ruby-base' value of the 'display' property is defined in CSS Ruby Layout. [CSSRUBY]

The following features are defined in CSS Table: [CSSTABLE]

The 'border-spacing' property
The 'border-collapse' property
The 'table-cell', 'table-row', 'table-caption', and 'table' values of the 'display' property

The following features are defined in CSS Text: [CSSTEXT]

The 'text-transform' property
The 'white-space' property
The 'text-align' property
The 'letter-spacing' property

The following features are defined in CSS Writing Modes: [CSSWM]

The 'direction' property
The 'unicode-bidi' property
The block flow direction, block size, inline size, block-start, block-end, inline-start, inline-end, line-left, and line-right concepts

The following features are defined in CSS Basic User Interface: [CSSUI]

The 'outline' property
The 'cursor' property
The 'appearance' property

The algorithm to update animations and send events is defined in Web Animations. [WEBANIMATIONS].

Implementations that support scripting must support the CSS Object Model. The following features and terms are defined in the CSSOM specifications: [CSSOM] [CSSOMVIEW]

The following features and terms are defined in CSS Syntax: [CSSSYNTAX]

The following terms are defined in Selectors: [SELECTORS]

The following features are defined in CSS Values and Units: [CSSVALUES]

<length>
The 'em' unit
The 'ex' unit
The 'vw' unit
The 'in' unit
The 'px' unit
The 'attr()' function
The math functions

The term style attribute is defined in CSS Style Attributes. [CSSATTR]

The following terms are defined in the CSS Cascading and Inheritance: [CSSCASCADE]

The CanvasRenderingContext2D object's use of fonts depends on the features described in the CSS Fonts and Font Loading specifications, including in particular FontFace objects and the font source concept. [CSSFONTS] [CSSFONTLOAD]

The following interfaces and terms are defined in Geometry Interfaces: [GEOMETRY]

DOMMatrix interface, and associated m11 element, m12 element, m21 element, m22 element, m41 element, and m42 element
DOMMatrix2DInit and DOMMatrixInit dictionaries
The create a DOMMatrix from a dictionary and create a DOMMatrix from a 2D dictionary algorithms for DOMMatrix2DInit or DOMMatrixInit

Intersection Observer

The following term is defined in Intersection Observer: [INTERSECTIONOBSERVER]

run the update intersection observations steps

WebGL

The following interfaces are defined in the WebGL specifications: [WEBGL]

WebVTT

Implementations may support WebVTT as a text track format for subtitles, captions, metadata, etc., for media resources. [WEBVTT]

The following terms, used in this specification, are defined in WebVTT:

The WebSocket protocol

The following terms are defined in Fetch: [FETCH]

establish a WebSocket connection

The following terms are defined in The WebSocket protocol: [WSP]

the WebSocket connection is established
extensions in use
subprotocol in use
a WebSocket message has been received
send a WebSocket Message
fail the WebSocket connection
close the WebSocket connection
start the WebSocket closing handshake
the WebSocket closing handshake is started
the WebSocket connection is closed (possibly cleanly)
the WebSocket connection close code
the WebSocket connection close reason
Sec-WebSocket-Protocol field

ARIA

The role attribute is defined in Accessible Rich Internet Applications (ARIA), as are the following roles: [ARIA]

In addition, the following aria-* content attributes are defined in ARIA: [ARIA]

Finally, the following terms are defined ARIA: [ARIA]

accessible name

Content Security Policy

The following terms are defined in Content Security Policy: [CSP]

Content Security Policy
Content Security Policy directive
CSP list
The Content Security Policy syntax
enforce the policy
The parse a serialized Content Security Policy algorithm
The Initialize a global object's CSP list algorithm
The Initialize a Document's CSP list algorithm
The Should element's inline behavior be blocked by Content Security Policy? algorithm
The Should navigation request of type from source in target be blocked by Content Security Policy? algorithm
The Should navigation response to navigation request of type from source in target be blocked by Content Security Policy? algorithm
The report-uri directive
The EnsureCSPDoesNotBlockStringCompilation abstract operation
The Is base allowed for Document? algorithm
The frame-ancestors directive
The sandbox directive
The Should element be blocked a priori by Content Security Policy? algorithm
The contains a header-delivered Content Security Policy property.

Service Workers

The following terms are defined in Service Workers: [SW]

Secure Contexts

The following algorithm is defined in Secure Contexts: [SECURE-CONTEXTS]

Is environment settings object a secure context?

Feature Policy

The following terms are defined in Feature Policy: [FEATUREPOLICY]

feature policy
feature policy
container policy
serialized feature policy
default allowlist
The creating a feature policy algorithm
The creating a feature policy from a response algorithm
The Is feature enabled by policy for origin algorithm
The Process feature policy attributes algorithm

Payment Request API

The following feature is defined in Payment Request API: [PAYMENTREQUEST]

PaymentRequest interface

MathML

While support for MathML as a whole is not required by this specification (though it is encouraged, at least for Web browsers), certain features depend upon small parts of MathML being implemented. [MATHML]

The following features are defined in Mathematical Markup Language (MathML):

SVG

While support for SVG as a whole is not required by this specification (though it is encouraged, at least for Web browsers), certain features depend upon parts of SVG being implemented.

User agents that implement SVG must implement the SVG 2 specification, and not any earlier revisions.

The following features are defined in the SVG 2 specification: [SVG]

SVGElement interface
SVGImageElement interface
SVGScriptElement interface
SVGSVGElement interface
SVG a element
SVG desc element
SVG foreignObject element
SVG image element
SVG script element
SVG svg element
SVG title element
SVG use element

Filter Effects

The following feature is defined in Filter Effects: [FILTERS]

<filter-function-list>

Worklets

The following feature is defined in Worklets: [WORKLETS]

Cooperative Scheduling of Background Tasks

The following features is defined in Cooperative Scheduling of Background Tasks: [REQUESTIDLECALLBACK]

This specification does not require support of any particular network protocol, style sheet language, scripting language, or any of the DOM specifications beyond those required in the list above. However, the language described by this specification is biased towards CSS as the styling language, JavaScript as the scripting language, and HTTP as the network protocol, and several features assume that those languages and protocols are in use.

A user agent that implements the HTTP protocol must implement HTTP State Management Mechanism (Cookies) as well. [HTTP] [COOKIES]

This specification might have certain additional requirements on character encodings, image formats, audio formats, and video formats in the respective sections.

2.1.10 Extensibility

Vendor-specific proprietary user agent extensions to this specification are strongly discouraged. Documents must not use such extensions, as doing so reduces interoperability and fragments the user base, allowing only users of specific user agents to access the content in question.

All extensions must be defined so that the use of extensions neither contradicts nor causes the non-conformance of functionality defined in the specification.

When vendor-neutral extensions to this specification are needed, either this specification can be updated accordingly, or an extension specification can be written that overrides the requirements in this specification. When someone applying this specification to their activities decides that they will recognize the requirements of such an extension specification, it becomes an applicable specification for the purposes of conformance requirements in this specification.

Someone could write a specification that defines any arbitrary byte stream as conforming, and then claim that their random junk is conforming. However, that does not mean that their random junk actually is conforming for everyone's purposes: if someone else decides that that specification does not apply to their work, then they can quite legitimately say that the aforementioned random junk is just that, junk, and not conforming at all. As far as conformance goes, what matters in a particular community is what that community agrees is applicable.

User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them.

When support for a feature is disabled (e.g. as an emergency measure to mitigate a security problem, or to aid in development, or for performance reasons), user agents must act as if they had no support for the feature whatsoever, and as if the feature was not mentioned in this specification. For example, if a particular feature is accessed via an attribute in a Web IDL interface, the attribute itself would be omitted from the objects that implement that interface — leaving the attribute on the object but making it return null or throw an exception is insufficient.

2.1.11 Interactions with XPath and XSLT

Implementations of XPath 1.0 that operate on HTML documents parsed or created in the manners described in this specification (e.g. as part of the document.evaluate() API) must act as if the following edit was applied to the XPath 1.0 specification.

This change is a willful violation of the XPath 1.0 specification, motivated by desire to have implementations be compatible with legacy content while still supporting the changes that this specification introduces to HTML regarding which namespace is used for HTML elements. [XPATH10]

XSLT 1.0 processors outputting to a DOM when the output method is "html" (either explicitly or via the defaulting rule in XSLT 1.0) are affected as follows:

If the transformation program outputs an element in no namespace, the processor must, prior to constructing the corresponding DOM element node, change the namespace of the element to the HTML namespace, ASCII-lowercase the element's local name, and ASCII-lowercase the names of any non-namespaced attributes on the element.

This requirement is a willful violation of the XSLT 1.0 specification, required because this specification changes the namespaces and case-sensitivity rules of HTML in a manner that would otherwise be incompatible with DOM-based XSLT transformations. (Processors that serialize the output are unaffected.) [XSLT10]

This specification does not specify precisely how XSLT processing interacts with the HTML parser infrastructure (for example, whether an XSLT processor acts as if it puts any elements into a stack of open elements). However, XSLT processors must stop parsing if they successfully complete, and must set the current document readiness first to "interactive" and then to "complete" if they are aborted.

This specification does not specify how XSLT interacts with the navigation algorithm, how it fits in with the event loop, nor how error pages are to be handled (e.g. whether XSLT errors are to replace an incremental XSLT output, or are rendered inline, etc).

2.2 Case-sensitivity and string comparison

Comparing two strings in a case-sensitive manner means comparing them exactly, code point for code point.

Except where otherwise stated, string comparisons must be performed in a case-sensitive manner.

A string pattern is a prefix match for a string s when pattern is not longer than s and truncating s to pattern's length leaves the two strings as matches of each other.

2.3 Policy-controlled features

2.4 Common microsyntaxes

There are various places in HTML that accept particular data types, such as dates or numbers. This section describes what the conformance criteria for content in those formats is, and how to parse them.

Implementors are strongly urged to carefully examine any third-party libraries they might consider using to implement the parsing of syntaxes described below. For example, date libraries are likely to implement error handling behavior that differs from what is required in this specification, since error-handling behavior is often not defined in specifications that describe date syntaxes similar to those used in this specification, and thus implementations tend to vary greatly in how they handle errors.

2.4.1 Common parser idioms

Some of the micro-parsers described below follow the pattern of having an input variable that holds the string being parsed, and having a position variable pointing at the next character to parse in input.

2.4.2 Boolean attributes

A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

2.4.3 Keywords and enumerated attributes

Some attributes are defined as taking one of a finite set of keywords. Such attributes are called enumerated attributes. The keywords are each defined to map to a particular state (several keywords might map to the same state, in which case some of the keywords are synonyms of each other; additionally, some of the keywords can be said to be non-conforming, and are only in the specification for historical reasons). In addition, two default states can be given. The first is the invalid value default, the second is the missing value default.

If an enumerated attribute is specified, the attribute's value must be an ASCII case-insensitive match for one of the given keywords that are not said to be non-conforming, with no leading or trailing whitespace.

When the attribute is specified, if its value is an ASCII case-insensitive match for one of the given keywords then that keyword's state is the state that the attribute represents. If the attribute value matches none of the given keywords, but the attribute has an invalid value default, then the attribute represents that state. Otherwise, there is no default, and invalid values mean that there is no state represented.

When the attribute is not specified, if there is a missing value default state defined, then that is the state represented by the (missing) attribute. Otherwise, the absence of the attribute means that there is no state represented.

2.4.4 Numbers

2.4.4.1 Signed integers

A string is a valid integer if it consists of one or more ASCII digits, optionally prefixed with a U+002D HYPHEN-MINUS character (-).

A valid integer without a U+002D HYPHEN-MINUS (-) prefix represents the number that is represented in base ten by that string of digits. A valid integer with a U+002D HYPHEN-MINUS (-) prefix represents the number represented in base ten by the string of digits that follows the U+002D HYPHEN-MINUS, subtracted from zero.

The rules for parsing integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either an integer or an error.

2.4.4.2 Non-negative integers

A string is a valid non-negative integer if it consists of one or more ASCII digits.

A valid non-negative integer represents the number that is represented in base ten by that string of digits.

The rules for parsing non-negative integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either zero, a positive integer, or an error.

2.4.4.3 Floating-point numbers

A valid floating-point number represents the number obtained by multiplying the significand by ten raised to the power of the exponent, where the significand is the first number, interpreted as base ten (including the decimal point and the number after the decimal point, if any, and interpreting the significand as a negative number if the whole string starts with a U+002D HYPHEN-MINUS character (-) and the number is not zero), and where the exponent is the number after the E, if any (interpreted as a negative number if there is a U+002D HYPHEN-MINUS character (-) between the E and the number and the number is not zero, or else ignoring a U+002B PLUS SIGN character (+) between the E and the number if there is one). If there is no E, then the exponent is treated as zero.

The best representation of the number n as a floating-point number is the string obtained from running ToString(n). The abstract operation ToString is not uniquely determined. When there are multiple possible strings that could be obtained from ToString for a particular value, the user agent must always return the same string for that value (though it may differ from the value used by other user agents).

The rules for parsing floating-point number values are as given in the following algorithm. This algorithm must be aborted at the first step that returns something. This algorithm will return either a number or an error.

2.4.4.4 Percentages and lengths

The rules for parsing dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than or equal to 0.0, or failure; if a number is returned, then it is further categorized as either a percentage or a length.

The current dimension value, given value, input, and position, is determined as follows:

2.4.4.5 Non-zero percentages and lengths

The rules for parsing nonzero dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than 0.0, or an error; if a number is returned, then it is further categorized as either a percentage or a length.

2.4.4.6 Lists of floating-point numbers

A valid list of floating-point numbers is a number of valid floating-point numbers separated by U+002C COMMA characters, with no other characters (e.g. no ASCII whitespace). In addition, there might be restrictions on the number of floating-point numbers that can be given, or on the range of values allowed.

2.4.4.7 Lists of dimensions

The rules for parsing a list of dimensions are as follows. These rules return a list of zero or more pairs consisting of a number and a unit, the unit being one of percentage, relative, and absolute.

2.4.5 Dates and times

In the algorithms below, the number of days in month month of year year is: 31 if month is 1, 3, 5, 7, 8, 10, or 12; 30 if month is 4, 6, 9, or 11; 29 if month is 2 and year is a number divisible by 400, or if year is a number divisible by 4 but not by 100; and 28 otherwise. This takes into account leap years in the Gregorian calendar. [GREGORIAN]

When ASCII digits are used in the date and time syntaxes defined in this section, they express numbers in base ten.

While the formats described here are intended to be subsets of the corresponding ISO8601 formats, this specification defines parsing rules in much more detail than ISO8601. Implementors are therefore encouraged to carefully examine any date parsing libraries before using them to implement the parsing rules described below; ISO8601 libraries might not parse dates and times in exactly the same manner. [ISO8601]

Where this specification refers to the proleptic Gregorian calendar, it means the modern Gregorian calendar, extrapolated backwards to year 1. A date in the proleptic Gregorian calendar, sometimes explicitly referred to as a proleptic-Gregorian date, is one that is described using that calendar even if that calendar was not in use at the time (or place) in question. [GREGORIAN]

2.4.5.1 Months

A month consists of a specific proleptic-Gregorian date with no time-zone information and no date information beyond a year and a month. [GREGORIAN]

A string is a valid month string representing a year year and month month if it consists of the following components in the given order:

The rules to parse a month string are as follows. This will return either a year and month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

The rules to parse a month component, given an input string and a position, are as follows. This will return either a year and a month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.2 Dates

A string is a valid date string representing a year year, month month, and day day if it consists of the following components in the given order:

The rules to parse a date string are as follows. This will return either a date, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

The rules to parse a date component, given an input string and a position, are as follows. This will return either a year, a month, and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.3 Yearless dates

A yearless date consists of a Gregorian month and a day within that month, but with no associated year. [GREGORIAN]

A string is a valid yearless date string representing a month month and a day day if it consists of the following components in the given order:

In other words, if the month is "02", meaning February, then the day can be 29, as if the year was a leap year.

The rules to parse a yearless date string are as follows. This will return either a month and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

The rules to parse a yearless date component, given an input string and a position, are as follows. This will return either a month and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.4 Times

A time consists of a specific time with no time-zone information, consisting of an hour, a minute, a second, and a fraction of a second.

A string is a valid time string representing an hour hour, a minute minute, and a second second if it consists of the following components in the given order:

The rules to parse a time string are as follows. This will return either a time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

The rules to parse a time component, given an input string and a position, are as follows. This will return either an hour, a minute, and a second, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.5 Local dates and times

A local date and time consists of a specific proleptic-Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, but expressed without a time zone. [GREGORIAN]

A string is a valid local date and time string representing a date and time if it consists of the following components in the given order:

A string is a valid normalized local date and time string representing a date and time if it consists of the following components in the given order:

The rules to parse a local date and time string are as follows. This will return either a date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.6 Time zones

A string is a valid time-zone offset string representing a time-zone offset if it consists of either:

This format allows for time-zone offsets from -23:59 to +23:59. Right now, in practice, the range of offsets of actual time zones is -12:00 to +14:00, and the minutes component of offsets of actual time zones is always either 00, 30, or 45. There is no guarantee that this will remain so forever, however, since time zones are used as political footballs and are thus subject to very whimsical policy decisions.

See also the usage notes and examples in the global date and time section below for details on using time-zone offsets with historical times that predate the formation of formal time zones.

The rules to parse a time-zone offset string are as follows. This will return either a time-zone offset, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

The rules to parse a time-zone offset component, given an input string and a position, are as follows. This will return either time-zone hours and time-zone minutes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.7 Global dates and times

A global date and time consists of a specific proleptic-Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, expressed with a time-zone offset, consisting of a signed number of hours and minutes. [GREGORIAN]

A string is a valid global date and time string representing a date, time, and a time-zone offset if it consists of the following components in the given order:

Times in dates before the formation of UTC in the mid twentieth century must be expressed and interpreted in terms of UT1 (contemporary Earth solar time at the 0° longitude), not UTC (the approximation of UT1 that ticks in SI seconds). Time before the formation of time zones must be expressed and interpreted as UT1 times with explicit time zones that approximate the contemporary difference between the appropriate local time and the time observed at the location of Greenwich, London.

The rules to parse a global date and time string are as follows. This will return either a time in UTC, with associated time-zone offset information for round-tripping or display purposes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.8 Weeks

A week consists of a week-year number and a week number representing a seven-day period starting on a Monday. Each week-year in this calendaring system has either 52 or 53 such seven-day periods, as defined below. The seven-day period starting on the Gregorian date Monday December 29th 1969 (1969-12-29) is defined as week number 1 in week-year 1970. Consecutive weeks are numbered sequentially. The week before the number 1 week in a week-year is the last week in the previous week-year, and vice versa. [GREGORIAN]

A week-year with a number year has 53 weeks if it corresponds to either a year year in the proleptic Gregorian calendar that has a Thursday as its first day (January 1st), or a year year in the proleptic Gregorian calendar that has a Wednesday as its first day (January 1st) and where year is a number divisible by 400, or a number divisible by 4 but not by 100. All other week-years have 52 weeks.

The week number of the last day of a week-year with 53 weeks is 53; the week number of the last day of a week-year with 52 weeks is 52.

The week-year number of a particular day can be different than the number of the year that contains that day in the proleptic Gregorian calendar. The first week in a week-year y is the week that contains the first Thursday of the Gregorian year y.

For modern purposes, a week as defined here is equivalent to ISO weeks as defined in ISO 8601. [ISO8601]

A string is a valid week string representing a week-year year and week week if it consists of the following components in the given order:

The rules to parse a week string are as follows. This will return either a week-year number and week number, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.9 Durations

Since months and seconds are not comparable (a month is not a precise number of seconds, but is instead a period whose exact length depends on the precise day from which it is measured) a duration as defined in this specification cannot include months (or years, which are equivalent to twelve months). Only durations that describe a specific number of seconds can be described.

A string is a valid duration string representing a duration t if it consists of either of the following:

The rules to parse a duration string are as follows. This will return either a duration or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.5.10 Vaguer moments in time

A string is a valid date string with optional time if it is also one of the following:

The rules to parse a date or time string are as follows. The algorithm will return either a date, a time, a global date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

2.4.6 Colors

A simple color consists of three 8-bit numbers in the range 0..255, representing the red, green, and blue components of the color respectively, in the sRGB color space. [SRGB]

A string is a valid simple color if it is exactly seven characters long, and the first character is a U+0023 NUMBER SIGN character (#), and the remaining six characters are all ASCII hex digits, with the first two digits representing the red component, the middle two digits representing the green component, and the last two digits representing the blue component, in hexadecimal.

A string is a valid lowercase simple color if it is a valid simple color and doesn't use any characters in the range U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F.

The rules for parsing simple color values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a simple color or an error.

The rules for serializing simple color values given a simple color are as given in the following algorithm:

Some obsolete legacy attributes parse colors in a more complicated manner, using the rules for parsing a legacy color value, which are given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a simple color or an error.

2.4.7 Space-separated tokens

A set of space-separated tokens is a string containing zero or more words (known as tokens) separated by one or more ASCII whitespace, where words consist of any string of one or more characters, none of which are ASCII whitespace.

An unordered set of unique space-separated tokens is a set of space-separated tokens where none of the tokens are duplicated.

An ordered set of unique space-separated tokens is a set of space-separated tokens where none of the tokens are duplicated but where the order of the tokens is meaningful.

Sets of space-separated tokens sometimes have a defined set of allowed values. When a set of allowed values is defined, the tokens must all be from that list of allowed values; other values are non-conforming. If no such set of allowed values is provided, then all values are conforming.

How tokens in a set of space-separated tokens are to be compared (e.g. case-sensitively or not) is defined on a per-set basis.

2.4.8 Comma-separated tokens

A set of comma-separated tokens is a string containing zero or more tokens each separated from the next by a single U+002C COMMA character (,), where tokens consist of any string of zero or more characters, neither beginning nor ending with ASCII whitespace, nor containing any U+002C COMMA characters (,), and optionally surrounded by ASCII whitespace.

For instance, the string " a ,b,,d d " consists of four tokens: "a", "b", the empty string, and "d d". Leading and trailing whitespace around each token doesn't count as part of the token, and the empty string can be a token.

Sets of comma-separated tokens sometimes have further restrictions on what consists a valid token. When such restrictions are defined, the tokens must all fit within those restrictions; other values are non-conforming. If no such restrictions are specified, then all values are conforming.

2.4.9 References

A valid hash-name reference to an element of type type is a string consisting of a U+0023 NUMBER SIGN character (#) followed by a string which exactly matches the value of the name attribute of an element with type type in the same tree.

The rules for parsing a hash-name reference to an element of type type, given a context node scope, are as follows:

2.4.10 Media queries

A string is a valid media query list if it matches the <media-query-list> production of Media Queries. [MQ]

A string matches the environment of the user if it is the empty string, a string consisting of only ASCII whitespace, or is a media query list that matches the user's environment according to the definitions given in Media Queries. [MQ]

2.5 URLs

2.5.1 Terminology

A string is a valid non-empty URL if it is a valid URL string but it is not the empty string.

This specification defines the URL about:legacy-compat as a reserved, though unresolvable, about: URL, for use in DOCTYPEs in HTML documents when needed for compatibility with XML tools. [ABOUT]

This specification defines the URL about:html-kind as a reserved, though unresolvable, about: URL, that is used as an identifier for kinds of media tracks. [ABOUT]

The fallback base URL of a Document object document is the URL record obtained by running these steps:

The document base URL of a Document object is the absolute URL obtained by running these steps:

2.5.2 Parsing URLs

Parsing a URL is the process of taking a string and obtaining the URL record that it represents. While this process is defined in URL, the HTML standard defines a wrapper for convenience. [URL]

This wrapper is only useful when the character encoding for the URL parser has to match that of the document or environment settings object for legacy reasons. When that is not the case the URL parser can be used directly.

To parse a URL url, relative to either a document or environment settings object, the user agent must use the following steps. Parsing a URL either results in failure or a resulting URL string and resulting URL record.

2.5.3 Dynamic changes to base URLs

2.6 Fetching resources

2.6.1 Terminology

A response whose type is "basic", "cors", or "default" is CORS-same-origin. [FETCH]

To create a potential-CORS request, given a url, destination, corsAttributeState, and an optional same-origin fallback flag, run these steps:

2.6.2 Determining the type of a resource

The Content-Type metadata of a resource must be obtained and interpreted in a manner consistent with the requirements of MIME Sniffing. [MIMESNIFF]

The computed MIME type of a resource must be found in a manner consistent with the requirements given in MIME Sniffing. [MIMESNIFF]

It is imperative that the rules in MIME Sniffing be followed exactly. When a user agent uses different heuristics for content type detection than the server expects, security problems can occur. For more details, see MIME Sniffing. [MIMESNIFF]

2.6.3 Extracting character encodings from meta elements

The algorithm for extracting a character encoding from a meta element, given a string s, is as follows. It either returns a character encoding or nothing.

This algorithm is distinct from those in the HTTP specifications (for example, HTTP doesn't allow the use of single quotes and requires supporting a backslash-escape mechanism that is not supported by this algorithm). While the algorithm is used in contexts that, historically, were related to HTTP, the syntax as supported by implementations diverged some time ago. [HTTP]

2.6.4 CORS settings attributes

A CORS settings attribute is an enumerated attribute. The following table lists the keywords and states for the attribute — the keywords in the left column map to the states in the cell in the second column on the same row as the keyword.

Keyword	State	Brief description
`anonymous`	Anonymous	Requests for the element will have their mode set to "`cors`" and their credentials mode set to "`same-origin`".
`use-credentials`	Use Credentials	Requests for the element will have their mode set to "`cors`" and their credentials mode set to "`include`".

For module scripts, certain CORS settings attributes have been repurposed to have a slightly different meaning, wherein they only impact the request's credentials mode (since the mode is always "cors"). To perform this translation, we define the module script credentials mode for a given CORS settings attribute to be determined by switching on the attribute's state:

2.6.5 Referrer policy attributes

A referrer policy attribute is an enumerated attribute. Each referrer policy, including the empty string, is a keyword for this attribute, mapping to a state of the same name.

The impact of these states on the processing model of various fetches is defined in more detail throughout this specification, in Fetch, and in Referrer Policy. [FETCH] [REFERRERPOLICY]

2.6.6 Nonce attributes

A nonce content attribute represents a cryptographic nonce ("number used once") which can be used by Content Security Policy to determine whether or not a given fetch will be allowed to proceed. The value is text. [CSP]

Elements that have a nonce content attribute ensure that the crytographic nonce is only exposed to script (and not to side-channels like CSS attribute selectors) by extracting the value from the content attribute, moving it into an internal slot named [[CryptographicNonce]], and exposing it to script via the HTMLOrSVGElement interface mixin. Unless otherwise specified, the slot's value is the empty string.

The nonce IDL attribute must, on getting, return the value of this element's [[CryptographicNonce]]; and on setting, set this element's [[CryptographicNonce]] to the given value.

Note how the setter for the nonce IDL attribute does not update the corresponding content attribute. This, as well as the below setting of the nonce content attribute to the empty string when an element becomes browsing-context connected, is meant to prevent exfiltration of the nonce value through mechanisms that can easily read content attributes, such as selectors. Learn more in issue #2369, where this behavior was introduced.

2.7 Common DOM interfaces

2.7.1 Reflecting content attributes in IDL attributes

Some IDL attributes are defined to reflect a particular content attribute. This means that on getting, the IDL attribute returns the current value of the content attribute, and on setting, the IDL attribute changes the value of the content attribute to the given value.

In general, on getting, if the content attribute is not present, the IDL attribute must act as if the content attribute's value is the empty string; and on setting, if the content attribute is not present, it must first be added.

If a reflecting IDL attribute is a USVString attribute whose content attribute is defined to contain a URL, then on getting, if the content attribute is absent, the IDL attribute must return the empty string. Otherwise, the IDL attribute must parse the value of the content attribute relative to the element's node document and if that is successful, return the resulting URL string. If parsing fails, then the value of the content attribute must be returned instead, converted to a USVString. On setting, the content attribute must be set to the specified new value.

If a reflecting IDL attribute is a DOMString attribute whose content attribute is an enumerated attribute, and the IDL attribute is limited to only known values, then, on getting, the IDL attribute must return the conforming value associated with the state the attribute is in (in its canonical case), if any, or the empty string if the attribute is in a state that has no associated keyword value or if the attribute is not in a defined state (e.g. the attribute is missing and there is no missing value default). On setting, the content attribute must be set to the specified new value.

If a reflecting IDL attribute is a nullable DOMString attribute whose content attribute is an enumerated attribute, then, on getting, if the corresponding content attribute is in its missing value default then the IDL attribute must return null, otherwise, the IDL attribute must return the conforming value associated with the state the attribute is in (in its canonical case). On setting, if the new value is null, the content attribute must be removed, and otherwise, the content attribute must be set to the specified new value.

If a reflecting IDL attribute is a DOMString or USVString attribute but doesn't fall into any of the above categories, then the getting and setting must be done in a transparent, case-preserving manner.

If a reflecting IDL attribute is a boolean attribute, then on getting the IDL attribute must return true if the content attribute is set, and false if it is absent. On setting, the content attribute must be removed if the IDL attribute is set to false, and must be set to the empty string if the IDL attribute is set to true. (This corresponds to the rules for boolean content attributes.)

If a reflecting IDL attribute has a signed integer type (long) then, on getting, the content attribute must be parsed according to the rules for parsing signed integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, then the default value must be returned instead, or 0 if there is no default value. On setting, the given value must be converted to the shortest possible string representing the number as a valid integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute has a signed integer type (long) that is limited to only non-negative numbers then, on getting, the content attribute must be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or −1 if there is no default value. On setting, if the value is negative, the user agent must throw an "IndexSizeError" DOMException. Otherwise, the given value must be converted to the shortest possible string representing the number as a valid non-negative integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute has an unsigned integer type (unsigned long) then, on getting, the content attribute must be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range 0 to 2147483647 inclusive, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 0 if there is no default value. On setting, first, if the new value is in the range 0 to 2147483647, then let n be the new value, otherwise let n be the default value, or 0 if there is no default value; then, n must be converted to the shortest possible string representing the number as a valid non-negative integer and that string must be used as the new content attribute value.

If a reflecting IDL attribute has an unsigned integer type (unsigned long) that is limited to only non-negative numbers greater than zero, then the behavior is similar to the previous case, but zero is not allowed. On getting, the content attribute must first be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range 1 to 2147483647 inclusive, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 1 if there is no default value. On setting, if the value is zero, the user agent must throw an "IndexSizeError" DOMException. Otherwise, first, if the new value is in the range 1 to 2147483647, then let n be the new value, otherwise let n be the default value, or 1 if there is no default value; then, n must be converted to the shortest possible string representing the number as a valid non-negative integer and that string must be used as the new content attribute value.

If a reflecting IDL attribute has an unsigned integer type (unsigned long) that is limited to only non-negative numbers greater than zero with fallback, then the behavior is similar to the previous case, but disallowed values are converted to the default value. On getting, the content attribute must first be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range 1 to 2147483647 inclusive, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead. On setting, first, if the new value is in the range 1 to 2147483647, then let n be the new value, otherwise let n be the default value; then, n must be converted to the shortest possible string representing the number as a valid non-negative integer and that string must be used as the new content attribute value.

If a reflecting IDL attribute has an unsigned integer type (unsigned long) that is clamped to the range [min, max], then on getting, the content attribute must first be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is between min and max inclusive, the resulting value must be returned. If it fails, the default value must be returned. If it succeeds but the value is less than min, min must be returned. If it succeeds but the value is greater than max, max must be returned. On setting, it behaves the same as setting a regular reflected unsigned integer.

If a reflecting IDL attribute has a floating-point number type (double or

unrestricted
  double

), then, on getting, the content attribute must be parsed according to the rules for parsing floating-point number values, and if that is successful, the resulting value must be returned. If, on the other hand, it fails, or if the attribute is absent, the default value must be returned instead, or 0.0 if there is no default value. On setting, the given value must be converted to the best representation of the number as a floating-point number and then that string must be used as the new content attribute value.

If a reflecting IDL attribute has a floating-point number type (double or

unrestricted
  double

) that is limited to numbers greater than zero, then the behavior is similar to the previous case, but zero and negative values are not allowed. On getting, the content attribute must be parsed according to the rules for parsing floating-point number values, and if that is successful and the value is greater than 0.0, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 0.0 if there is no default value. On setting, if the value is less than or equal to zero, then the value must be ignored. Otherwise, the given value must be converted to the best representation of the number as a floating-point number and then that string must be used as the new content attribute value.

The values Infinity and Not-a-Number (NaN) values throw an exception on setting, as defined in Web IDL. [WEBIDL]

If a reflecting IDL attribute has the type DOMTokenList, then on getting it must return a DOMTokenList object whose associated element is the element in question and whose associated attribute's local name is the name of the attribute in question.

2.7.2 Collections

2.7.2.1 The HTMLAllCollection interface

The HTMLAllCollection interface is used for the legacy document.all attribute. It operates similarly to HTMLCollection; the main differences are that it allows a staggering variety of different (ab)uses of its methods to all end up returning something, and that it can be called as a function as an alternative to property access.

To get the "all"-indexed element from an HTMLAllCollection collection given an index index, return the index^th element in collection, or null if there is no such index^th element.

To get the "all"-named element(s) from an HTMLAllCollection collection given a name name, perform the following steps:

To get the "all"-indexed or named element(s) from an HTMLAllCollection collection given nameOrIndex:

2.7.2.1.1 [[Call]] ( thisArgument, argumentsList )

The thisArgument is ignored, and thus code such as Function.prototype.call.call(document.all, null, "x") will still search for elements. (document.all.call does not exist, since document.all does not inherit from Function.prototype.)

2.7.2.2 The HTMLFormControlsCollection interface

The supported property names consist of the non-empty values of all the id and name attributes of all the elements represented by the collection, in tree order, ignoring later duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

Members of the RadioNodeList interface inherited from the NodeList interface must behave as they would on a NodeList object.

The value IDL attribute on the RadioNodeList object, on getting, must return the value returned by running the following steps:

2.7.2.3 The HTMLOptionsCollection interface

The HTMLOptionsCollection interface is used for collections of option elements. It is always rooted on a select element and has attributes and methods that manipulate that element's descendants.

On setting, the behavior depends on whether the new value is equal to, greater than, or less than the number of nodes represented by the collection at that time. If the number is the same, then setting the attribute must do nothing. If the new value is greater, then n new option elements with no attributes and no child nodes must be appended to the select element on which the HTMLOptionsCollection is rooted, where n is the difference between the two numbers (new value minus old value). Mutation events must be fired as if a DocumentFragment containing the new option elements had been inserted. If the new value is lower, then the last n nodes in the collection must be removed from their parent nodes, where n is the difference between the two numbers (old value minus new value).

Setting length never removes or adds any optgroup elements, and never adds new children to existing optgroup elements (though it can remove children from them).

The add(element, before) method must act according to the following algorithm:

The selectedIndex IDL attribute must act like the identically named attribute on the select element on which the HTMLOptionsCollection is rooted

2.7.3 The DOMStringList interface

The DOMStringList interface is a non-fashionable retro way of representing a list of strings.

New APIs must use sequence<DOMString> or equivalent rather than DOMStringList.

The length attribute's getter must return this DOMStringList object's associated list's size.

The item(index) method, when invoked, must return the indexth item in this DOMStringList object's associated list, or null if index plus one is greater than this DOMStringList object's associated list's size.

The contains(string) method, when invoked, must return true if this DOMStringList object's associated list contains string, and false otherwise.

2.8 Safe passing of structured data

This section uses the terminology and typographic conventions from the JavaScript specification. [JAVASCRIPT]

2.8.1 Serializable objects

Serializable objects support being serialized, and later deserialized, in a way that is independent of any given JavaScript Realm. This allows them to be stored on disk and later restored, or cloned across agent and even agent cluster boundaries.

It is up to the definition of individual platform objects to determine what data is serialized and deserialized by these steps. Typically the steps are very symmetric.

The [Serializable] extended attribute must take no arguments, and must not appear on anything other than an interface. It must appear only once on an interface. It must not be used on a callback interface. If it appears on a partial interface or an interface that is really a mixin, then it must also appear on the original or mixed-in-to interface, and any supplied serialization steps and deserialization steps for the partial interface or mixin should be understood as being appended to those of the original or mixed-in-to interface.

Objects defined in the JavaScript specification are handled by the StructuredSerialize abstract operation directly.

Originally, this specification defined the concept of "cloneable objects", which could be cloned from one JavaScript Realm to another. However, to better specify the behavior of certain more complex situations, the model was updated to make the serialization and deserialization explicit.

2.8.2 Transferable objects

Transferable objects support being transferred across agents. Transferring is effectively recreating the object while sharing a reference to the underlying data and then detaching the object being transferred. This is useful to transfer ownership of expensive resources. Not all objects are transferable objects and not all aspects of objects that are transferable objects are necessarily preserved when transferred.

Transferring is an irreversible and non-idempotent operation. Once an object has been transferred, it cannot be transferred, or indeed used, again.

It is up to the definition of individual platform objects to determine what data is transferred by these steps. Typically the steps are very symmetric.

The [Transferable] extended attribute must take no arguments, and must not appear on anything other than an interface. It must appear only once on an interface. It must not be used on a callback interface. If it appears on a partial interface or an interface that is really a mixin, then it must also appear on the original or mixed-in-to interface, and any supplied transfer steps and transfer-receiving steps for the partial interface or mixin should be understood as being appended to those of the original or mixed-in-to interface.

Platform objects that are transferable objects have a [[Detached]] internal slot. This is used to ensure that once a platform object has been transferred, it cannot be transferred again.

Objects defined in the JavaScript specification are handled by the StructuredSerializeWithTransfer abstract operation directly.

2.8.3 StructuredSerializeInternal ( value, forStorage [ , memory ] )

The StructuredSerializeInternal abstract operation takes as input a JavaScript value value and serializes it to a Realm-independent form, represented here as a Record. This serialized form has all the information necessary to later deserialize into a new JavaScript value in a different Realm.

This process can throw an exception, for example when trying to serialize un-serializable objects.

2.8.4 StructuredSerialize ( value )

2.8.5 StructuredSerializeForStorage ( value )

2.8.6 StructuredDeserialize ( serialized, targetRealm [ , memory ] )

This process can throw an exception, for example when trying to allocate memory for the new objects (especially ArrayBuffer objects).

2.8.7 StructuredSerializeWithTransfer ( value, transferList )

2.8.8 StructuredDeserializeWithTransfer ( serializeWithTransferResult, targetRealm )

2.8.9 Performing serialization and transferring from other specifications

Other specifications may use the abstract operations defined here. The following provides some guidance on when each abstract operation is typically useful, with examples.

In general, call sites may pass in Web IDL values instead of JavaScript values; this is to be understood to perform an implicit conversion to the JavaScript value before invoking these algorithms.

This specification used to define a "structured clone" algorithm, and more recently a StructuredClone abstract operation. However, in practice all known uses of it were better served by separate serialization and deserialization steps, so it was removed.

Call sites that are not invoked as a result of author code synchronously calling into a user agent method must take care to properly prepare to run script and prepare to run a callback before invoking StructuredSerialize, StructuredSerializeForStorage, or StructuredSerializeWithTransfer abstract operations, if they are being performed on arbitrary objects. This is necessary because the serialization process can invoke author-defined accessors as part of its final deep-serialization steps, and these accessors could call into operations that rely on the entry and incumbent concepts being properly set up.

In contrast, a hypothetical API that used StructuredSerialize to serialize some author-supplied object periodically, directly from a task on the event loop, would need to ensure it performs the appropriate preparations beforehand. As of this time, we know of no such APIs on the platform; usually it is simpler to perform the serialization ahead of time, as a synchronous consequence of author code.

3 Semantics, structure, and APIs of HTML documents

3.1 Documents

Every XML and HTML document in an HTML UA is represented by a Document object. [DOM]

The Document object's URL is defined in DOM. It is initially set when the Document object is created, but can change during the lifetime of the Document object; for example, it changes when the user navigates to a fragment on the page and when the pushState() method is called with a new URL. [DOM]

Interactive user agents typically expose the Document object's URL in their user interface. This is the primary mechanism by which a user can tell if a site is attempting to impersonate another.

The document's referrer is a string (representing a URL) that can be set when the Document is created. If it is not explicitly set, then its value is the empty string.

3.1.1 The Document object

DOM defines a Document interface, which this specification extends significantly.

The Document has an HTTPS state (an HTTPS state value), initially "none", which represents the security properties of the network channel used to deliver the Document's data.

The Document has a CSP list, which is a CSP list containing all of the Content Security Policy objects active for the document. The list is empty unless otherwise specified.

The Document has a feature policy, which is a feature policy, which is initially empty.

3.1.2 The DocumentOrShadowRoot interface

3.1.3 Resource metadata management

The cookie attribute represents the cookies of the resource identified by the document's URL.

A Document object that falls into one of the following conditions is a cookie-averse Document object:

Since the attribute is accessible across frames, the path restrictions on cookies are only a tool to help manage which cookies are sent to which parts of the site, and are not in any way a security feature.

The attribute's getter and setter synchronously access shared state. Since there is no locking mechanism, other browsing contexts in a multiprocess user agent can modify cookies while scripts are running. A site could, for instance, try to read a cookie, increment its value, then write it back out, using the new value of the cookie as a unique identifier for the session; if the site does this twice in two different browser windows at the same time, it might end up using the same "unique" identifier for both sessions, with potentially disastrous effects.

The lastModified attribute, on getting, must return the date and time of the Document's source file's last modification, in the user's local time zone, in the following format:

All the numeric components above, other than the year, must be given as two ASCII digits representing the number in base ten, zero-padded if necessary. The year must be given as the shortest possible string of four or more ASCII digits representing the number in base ten, zero-padded if necessary.

The Document's source file's last modification date and time must be derived from relevant features of the networking protocols used, e.g. from the value of the HTTP `Last-Modified` header of the document, or from metadata in the file system for local files. If the last modification date and time are not known, the attribute must return the current date and time in the above format.

Each document has a current document readiness. When a Document object is created, it must have its current document readiness set to the string "loading" if the document is associated with an HTML parser, an XML parser, or an XSLT processor, and to the string "complete" otherwise. Various algorithms during page loading affect this value. When the value is set, the user agent must fire an event named readystatechange at the Document object.

3.1.4 DOM tree accessors

The html element of a document is its document element, if it's an html element, and null otherwise.

The head element of a document is the first head element that is a child of the html element, if there is one, or null otherwise.

The head attribute, on getting, must return the head element of the document (a head element or null).

The title element of a document is the first title element in the document (in tree order), if there is one, or null otherwise.

On setting, the steps corresponding to the first matching condition in the following list must be run:

The body element of a document is the first of the html element's children that is either a body element or a frameset element, or null if there is no such element.

The body attribute, on getting, must return the body element of the document (either a body element, a frameset element, or null). On setting, the following algorithm must be run:

The value returned by the body getter is not always the one passed to the setter.

The images attribute must return an HTMLCollection rooted at the Document node, whose filter matches only img elements.

The embeds attribute must return an HTMLCollection rooted at the Document node, whose filter matches only embed elements.

The plugins attribute must return the same object as that returned by the embeds attribute.

The links attribute must return an HTMLCollection rooted at the Document node, whose filter matches only a elements with href attributes and area elements with href attributes.

The forms attribute must return an HTMLCollection rooted at the Document node, whose filter matches only form elements.

The scripts attribute must return an HTMLCollection rooted at the Document node, whose filter matches only script elements.

The getElementsByName(name) method takes a string name, and must return a live NodeList containing all the HTML elements in that document that have a name attribute whose value is equal to the name argument (in a case-sensitive manner), in tree order. When the method is invoked on a Document object again with the same argument, the user agent may return the same as the object returned by the earlier call. In other cases, a new NodeList object must be returned.

The currentScript attribute, on getting, must return the value to which it was most recently set. When the Document is created, the currentScript must be initialized to null.

This API has fallen out of favor in the implementer and standards community, as it globally exposes script or SVG script elements. As such, it is not available in newer contexts, such as when running module scripts or when running scripts in a shadow tree. We are looking into creating a new solution for identifying the running script in such contexts, which does not make it globally available: see issue #1013.

The Document interface supports named properties. The supported property names of a Document object document at any moment consist of the following, in tree order according to the element that contributed them, ignoring later duplicates, and with values from id attributes coming before values from name attributes when the same element contributes both:

Named elements with the name name, for the purposes of the above algorithm, are those that are either:

The dir attribute on the Document interface is defined along with the dir content attribute.

3.2 Elements

3.2.1 Semantics

Elements, attributes, and attribute values in HTML are defined (by this specification) to have certain meanings (semantics). For example, the ol element represents an ordered list, and the lang attribute represents the language of the content.

These definitions allow HTML processors, such as Web browsers or search engines, to present and use documents and applications in a wide variety of contexts that the author might not have considered.

Authors must not use elements, attributes, or attribute values for purposes other than their appropriate intended semantic purpose, as doing so prevents software from correctly processing the page.

Authors must not use elements, attributes, or attribute values that are not permitted by this specification or other applicable specifications, as doing so makes it significantly harder for the language to be extended in the future.

Through scripting and using other mechanisms, the values of attributes, text, and indeed the entire structure of the document may change dynamically while a user agent is processing it. The semantics of a document at an instant in time are those represented by the state of the document at that instant in time, and the semantics of a document can therefore change over time. User agents must update their presentation of the document as this occurs.

HTML has a progress element that describes a progress bar. If its "value" attribute is dynamically updated by a script, the UA would update the rendering to show the progress changing.

3.2.2 Elements in the DOM

The nodes representing HTML elements in the DOM must implement, and expose to scripts, the interfaces listed for them in the relevant sections of this specification. This includes HTML elements in XML documents, even when those documents are in another context (e.g. inside an XSLT transform).

Elements in the DOM represent things; that is, they have intrinsic meaning, also known as semantics.

Elements can be referenced (referred to) in some way, either explicitly or implicitly. One way that an element in the DOM can be explicitly referenced is by giving an id attribute to the element, and then creating a hyperlink with that id attribute's value as the fragment for the hyperlink's href attribute value. Hyperlinks are not necessary for a reference, however; any manner of referring to the element in question will suffice.

The basic interface, from which all the HTML elements' interfaces inherit, and which must be used by elements that have no additional requirements, is the HTMLElement interface.

The HTMLElement interface holds methods and attributes related to a number of disparate features, and the members of this interface are therefore described in various different sections of this specification.

Features shared between HTML and SVG elements use the HTMLOrSVGElement interface mixin: [SVG]

3.2.3 HTML element constructors

To support the custom elements feature, all HTML elements have special constructor behavior. This is indicated via the [HTMLConstructor] IDL extended attribute. It indicates that the interface object for the given interface will have a specific behavior when called, as defined in detail below.

The [HTMLConstructor] extended attribute must take no arguments, and must not appear on anything other than an interface. It must appear only once on an interface, and the interface must not be annotated with the [Constructor] or [NoInterfaceObject] extended attributes. (However, the interface may be annotated with [NamedConstructor]; there is no conflict there.) It must not be used on a callback interface.

Interface objects for interfaces annotated with the [HTMLConstructor] extended attribute must run the following steps as the function body behavior for both [[Call]] and [[Construct]] invocations of the corresponding JavaScript function object. When invoked with [[Call]], the NewTarget value is undefined, and so the algorithm below will immediately throw. When invoked with [[Construct]], the [[Construct]] newTarget parameter provides the NewTarget value.

In addition to the constructor behavior implied by [HTMLConstructor], some elements also have named constructors (which are really factory functions with a modified prototype property).

3.2.4 Element definitions

Each element in this specification has a definition that includes the following information:

This is then followed by a description of what the element represents, along with any additional normative conformance criteria that may apply to authors and implementations. Examples are sometimes also included.

3.2.4.1 Attributes

An attribute value is a string. Except where otherwise specified, attribute values on HTML elements may be any string value, including the empty string, and there is no restriction on what text can be specified in such attribute values.

3.2.5 Content models

Each element defined in this specification has a content model: a description of the element's expected contents. An HTML element must have contents that match the requirements described in the element's content model. The contents of an element are its children in the DOM.

ASCII whitespace is always allowed between elements. User agents represent these characters between elements in the source markup as Text nodes in the DOM. Empty Text nodes and Text nodes consisting of just sequences of those characters are considered inter-element whitespace.

Inter-element whitespace, comment nodes, and processing instruction nodes must be ignored when establishing whether an element's contents match the element's content model or not, and must be ignored when following algorithms that define document and element semantics.

Thus, an element A is said to be preceded or followed by a second element B if A and B have the same parent node and there are no other element nodes or Text nodes (other than inter-element whitespace) between them. Similarly, a node is the only child of an element if that element contains no other nodes other than inter-element whitespace, comment nodes, and processing instruction nodes.

Authors must not use HTML elements anywhere except where they are explicitly allowed, as defined for each element, or as explicitly required by other specifications. For XML compound documents, these contexts could be inside elements from other namespaces, if those elements are defined as providing the relevant contexts.

The Atom Syndication Format defines a content element. When its type attribute has the value xhtml, The Atom Syndication Format requires that it contain a single HTML div element. Thus, a div element is allowed in that context, even though this is not explicitly normatively stated by this specification. [ATOM]

3.2.5.1 The "nothing" content model

When an element's content model is nothing, the element must contain no Text nodes (other than inter-element whitespace) and no element nodes.

Most HTML elements whose content model is "nothing" are also, for convenience, void elements (elements that have no end tag in the HTML syntax). However, these are entirely separate concepts.

3.2.5.2 Kinds of content

Each element in HTML falls into zero or more categories that group elements with similar characteristics together. The following broad categories are used in this specification:

Some elements also fall into other categories, which are defined in other parts of this specification.

Sectioning content, heading content, phrasing content, embedded content, and interactive content are all types of flow content. Metadata is sometimes flow content. Metadata and interactive content are sometimes phrasing content. Embedded content is also a type of phrasing content, and sometimes is interactive content.

Other categories are also used for specific purposes, e.g. form controls are specified using a number of categories to define common requirements. Some elements have unique requirements and do not fit into any particular category.

3.2.5.2.1 Metadata content

Metadata content is content that sets up the presentation or behavior of the rest of the content, or that sets up the relationship of the document with other documents, or that conveys other "out of band" information.

Elements from other namespaces whose semantics are primarily metadata-related (e.g. RDF) are also metadata content.

3.2.5.2.2 Flow content

Most elements that are used in the body of documents and applications are categorized as flow content.

3.2.5.2.3 Sectioning content

3.2.5.2.4 Heading content

Heading content defines the header of a section (whether explicitly marked up using sectioning content elements, or implied by the heading content itself).

3.2.5.2.5 Phrasing content

Phrasing content is the text of the document, as well as elements that mark up that text at the intra-paragraph level. Runs of phrasing content form paragraphs.

Most elements that are categorized as phrasing content can only contain elements that are themselves categorized as phrasing content, not any flow content.

Text nodes and attribute values must consist of scalar values, excluding noncharacters, and controls other than ASCII whitespace. This specification includes extra constraints on the exact value of Text nodes and attribute values depending on their precise context.

3.2.5.2.6 Embedded content

Embedded content is content that imports another resource into the document, or content from another vocabulary that is inserted into the document.

Elements that are from namespaces other than the HTML namespace and that convey content but not metadata, are embedded content for the purposes of the content models defined in this specification. (For example, MathML, or SVG.)

Some embedded content elements can have fallback content: content that is to be used when the external resource cannot be used (e.g. because it is of an unsupported format). The element definitions state what the fallback is, if any.

3.2.5.2.7 Interactive content

Interactive content is content that is specifically intended for user interaction.

3.2.5.2.8 Palpable content

As a general rule, elements whose content model allows any flow content or phrasing content should have at least one node in its contents that is palpable content and that does not have the hidden attribute specified.

Palpable content makes an element non-empty by providing either some descendant non-empty text, or else something users can hear (audio elements) or view (video or img or canvas elements) or otherwise interact with (for example, interactive form controls).

This requirement is not a hard requirement, however, as there are many cases where an element can be empty legitimately, for example when it is used as a placeholder which will later be filled in by a script, or when the element is part of a template and would on most pages be filled in but on some pages is not relevant.

Conformance checkers are encouraged to provide a mechanism for authors to find elements that fail to fulfill this requirement, as an authoring aid.

3.2.5.2.9 Script-supporting elements

Script-supporting elements are those that do not represent anything themselves (i.e. they are not rendered), but are used to support scripts, e.g. to provide functionality for the user.

3.2.5.3 Transparent content models

Some elements are described as transparent; they have "transparent" in the description of their content model. The content model of a transparent element is derived from the content model of its parent element: the elements required in the part of the content model that is "transparent" are the same elements as required in the part of the content model of the parent of the transparent element in which the transparent element finds itself.

In some cases, where transparent elements are nested in each other, the process has to be applied iteratively.

When a transparent element has no parent, then the part of its content model that is "transparent" must instead be treated as accepting any flow content.

3.2.5.4 Paragraphs

The term paragraph as defined in this section is used for more than just the definition of the p element. The paragraph concept defined here is used to describe how to interpret documents. The p element is merely one of several ways of marking up a paragraph.

A paragraph is typically a run of phrasing content that forms a block of text with one or more sentences that discuss a particular topic, as in typography, but can also be used for more general thematic grouping. For instance, an address is also a paragraph, as is a part of a form, a byline, or a stanza in a poem.

Paragraphs in flow content are defined relative to what the document looks like without the a, ins, del, and map elements complicating matters, since those elements, with their hybrid content models, can straddle paragraph boundaries, as shown in the first two examples below.

Generally, having elements straddle paragraph boundaries is best avoided. Maintaining such markup can be difficult.

Let view be a view of the DOM that replaces all a, ins, del, and map elements in the document with their contents. Then, in view, for each run of sibling phrasing content nodes uninterrupted by other types of content, in an element that accepts content other than phrasing content as well as phrasing content, let first be the first node of the run, and let last be the last node of the run. For each such run that consists of at least one node that is neither embedded content nor inter-element whitespace, a paragraph exists in the original DOM from immediately before first to immediately after last. (Paragraphs can thus span across a, ins, del, and map elements.)

Conformance checkers may warn authors of cases where they have paragraphs that overlap each other (this can happen with object, video, audio, and canvas elements, and indirectly through elements in other namespaces that allow HTML to be further embedded therein, like SVG svg or MathML math).

The p element can be used to wrap individual paragraphs when there would otherwise not be any content other than phrasing content to separate the paragraphs from each other.

3.2.6 Global attributes

The following attributes are common to and may be specified on all HTML elements (even those not defined in this specification):

These attributes are only defined by this specification as attributes for HTML elements. When this specification refers to elements having these attributes, elements from namespaces that are not defined as having these attributes must not be considered as being elements with these attributes.

DOM standard defines the user agent requirements for the class, id, and slot attributes for any element in any namespace. [DOM]

When specified on HTML elements, the id attribute value must be unique amongst all the IDs in the element's tree and must contain at least one character. The value must not contain any ASCII whitespace.

Identifiers are opaque strings. Particular meanings should not be derived from the value of the id attribute.

There are no conformance requirements for the slot attribute specific to HTML elements.

The slot attribute is used to assign a slot to an element: an element with a slot attribute is assigned to the slot created by the slot element whose name attribute's value matches that slot attribute's value — but only if that slot element finds itself in the shadow tree whose root's host has the corresponding slot attribute value.

To enable assistive technology products to expose a more fine-grained interface than is otherwise possible with HTML elements and attributes, a set of annotations for assistive technology products can be specified (the ARIA role and aria-* attributes). [ARIA]

The attributes marked with an asterisk have a different meaning when specified on body elements as those elements expose event handlers of the Window object with the same names.

While these attributes apply to all elements, they are not useful on all elements. For example, only media elements will ever receive a volumechange event fired by the user agent.

Custom data attributes (e.g. data-foldername or data-msgid) can be specified on any HTML element, to store custom data, state, annotations, and similar, specific to the page.

In HTML documents, elements in the HTML namespace may have an xmlns attribute specified, if, and only if, it has the exact value "http://www.w3.org/1999/xhtml". This does not apply to XML documents.

In HTML, the xmlns attribute has absolutely no effect. It is basically a talisman. It is allowed merely to make migration to and from XML mildly easier. When parsed by an HTML parser, the attribute ends up in no namespace, not the "http://www.w3.org/2000/xmlns/" namespace like namespace declaration attributes in XML do.

In XML, an xmlns attribute is part of the namespace declaration mechanism, and an element cannot actually have an xmlns attribute in no namespace specified.

XML also allows the use of the xml:space attribute in the XML namespace on any element in an XML document. This attribute has no effect on HTML elements, as the default behavior in HTML is to preserve whitespace. [XML]

3.2.6.1 The title attribute

The title attribute represents advisory information for the element, such as would be appropriate for a tooltip. On a link, this could be the title or a description of the target resource; on an image, it could be the image credit or a description of the image; on a paragraph, it could be a footnote or commentary on the text; on a citation, it could be further information about the source; on interactive content, it could be a label for, or instructions for, use of the element; and so forth. The value is text.

Relying on the title attribute is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification (e.g., requiring a pointing device such as a mouse to cause a tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a modern phone or tablet).

If this attribute is omitted from an element, then it implies that the title attribute of the nearest ancestor HTML element with a title attribute set is also relevant to this element. Setting the attribute overrides this, explicitly stating that the advisory information of any ancestors is not relevant to this element. Setting the attribute to the empty string indicates that the element has no advisory information.

If the title attribute's value contains U+000A LINE FEED (LF) characters, the content is split into multiple lines. Each U+000A LINE FEED (LF) character represents a line break.

Some elements, such as link, abbr, and input, define additional semantics for the title attribute beyond the semantics described above.

The advisory information of an element is the value that the following algorithm returns, with the algorithm being aborted once a value is returned. When the algorithm returns the empty string, then there is no advisory information.

User agents should inform the user when elements have advisory information, otherwise the information would not be discoverable.

3.2.6.2 The lang and xml:lang attributes

The lang attribute (in no namespace) specifies the primary language for the element's contents and for any of the element's attributes that contain text. Its value must be a valid BCP 47 language tag, or the empty string. Setting the attribute to the empty string indicates that the primary language is unknown. [BCP47]

If these attributes are omitted from an element, then the language of this element is the same as the language of its parent element, if any.

Authors must not use the lang attribute in the XML namespace on HTML elements in HTML documents. To ease migration to and from XML, authors may specify an attribute in no namespace with no prefix and with the literal localname "xml:lang" on HTML elements in HTML documents, but such attributes must only be specified if a lang attribute in no namespace is also specified, and both attributes must have the same value when compared in an ASCII case-insensitive manner.

The attribute in no namespace with no prefix and with the literal localname "xml:lang" has no effect on language processing.

To determine the language of a node, user agents must look at the nearest ancestor element (including the element itself if the node is an element) that has a lang attribute in the XML namespace set or is an HTML element and has a lang in no namespace attribute set. That attribute specifies the language of the node (regardless of its value).

If node's inclusive ancestors do not have either attribute set, but there is a pragma-set default language set, then that is the language of the node. If there is no pragma-set default language set, then language information from a higher-level protocol (such as HTTP), if any, must be used as the final fallback language instead. In the absence of any such language information, and in cases where the higher-level protocol reports multiple languages, the language of the node is unknown, and the corresponding language tag is the empty string.

If the resulting value is not a recognized language tag, then it must be treated as an unknown language having the given language tag, distinct from all other languages. For the purposes of round-tripping or communicating with other services that expect language tags, user agents should pass unknown language tags through unmodified, and tagged as being BCP 47 language tags, so that subsequent services do not interpret the data as another type of language description. [BCP47]

Thus, for instance, an element with lang="xyzzy" would be matched by the selector :lang(xyzzy) (e.g. in CSS), but it would not be matched by :lang(abcde), even though both are equally invalid. Similarly, if a Web browser and screen reader working in unison communicated about the language of the element, the browser would tell the screen reader that the language was "xyzzy", even if it knew it was invalid, just in case the screen reader actually supported a language with that tag after all. Even if the screen reader supported both BCP 47 and another syntax for encoding language names, and in that other syntax the string "xyzzy" was a way to denote the Belarusian language, it would be incorrect for the screen reader to then start treating text as Belarusian, because "xyzzy" is not how Belarusian is described in BCP 47 codes (BCP 47 uses the code "be" for Belarusian).

If the resulting value is the empty string, then it must be interpreted as meaning that the language of the node is explicitly unknown.

User agents may use the element's language to determine proper processing or rendering (e.g. in the selection of appropriate fonts or pronunciations, for dictionary selection, or for the user interfaces of form controls such as date pickers).

The lang IDL attribute must reflect the lang content attribute in no namespace.

3.2.6.3 The translate attribute

The translate attribute is an enumerated attribute that is used to specify whether an element's attribute values and the values of its Text node children are to be translated when the page is localized, or whether to leave them unchanged.

The attribute's keywords are the empty string, yes, and no. The empty string and the yes keyword map to the yes state. The no keyword maps to the no state. In addition, there is a third state, the inherit state, which is the missing value default and the invalid value default.

When an element is in the translate-enabled state, the element's translatable attributes and the values of its Text node children are to be translated when the page is localized.

When an element is in the no-translate state, the element's attribute values and the values of its Text node children are to be left as-is when the page is localized, e.g. because the element contains a person's name or a name of a computer program.

Other specifications may define other attributes that are also translatable attributes. For example, ARIA would define the aria-label attribute as translatable.

The translate IDL attribute must, on getting, return true if the element's translation mode is translate-enabled, and false otherwise. On setting, it must set the content attribute's value to "yes" if the new value is true, and set the content attribute's value to "no" otherwise.

3.2.6.4 The dir attribute

The dir attribute specifies the element's text directionality. The attribute is an enumerated attribute with the following keywords and states:

The directionality of an element (any element, not just an HTML element) is either 'ltr' or 'rtl', and is determined as per the first appropriate set of steps from the following list:

Since the dir attribute is only defined for HTML elements, it cannot be present on elements from other namespaces. Thus, elements from other namespaces always just inherit their directionality from their parent element, or, if they don't have one, default to 'ltr'.

The directionality of an attribute of an HTML element, which is used when the text of that attribute is to be included in the rendering in some manner, is determined as per the first appropriate set of steps from the following list:

Authors are strongly encouraged to use the dir attribute to indicate text direction rather than using CSS, since that way their documents will continue to render correctly even in the absence of CSS (e.g. as interpreted by search engines).

3.2.6.5 The style attribute

In user agents that support CSS, the attribute's value must be parsed when the attribute is added or has its value changed, according to the rules given for style attributes. [CSSATTR]

Documents that use style attributes on any of their elements must still be comprehensible and usable if those attributes were removed.

In particular, using the style attribute to hide and show content, or to convey meaning that is otherwise not included in the document, is non-conforming. (To hide and show content, use the hidden attribute.)

3.2.6.6 Embedding custom non-visible data with the data-* attributes

A custom data attribute is an attribute in no namespace whose name starts with the string "data-", has at least one character after the hyphen, is XML-compatible, and contains no ASCII upper alphas.

All attribute names on HTML elements in HTML documents get ASCII-lowercased automatically, so the restriction on ASCII uppercase letters doesn't affect such documents.

Custom data attributes are intended to store custom data, state, annotations, and similar, private to the page or application, for which there are no more appropriate attributes or elements.

These attributes are not intended for use by software that is not known to the administrators of the site that uses the attributes. For generic extensions that are to be used by multiple independent tools, either this specification should be extended to provide the feature explicitly, or a technology like microdata should be used (with a standardized vocabulary).

Authors should carefully design such extensions so that when the attributes are ignored and any associated CSS dropped, the page is still usable.

User agents must not derive any implementation behavior from these attributes or values. Specifications intended for user agents must not define these attributes to have any meaningful values.

JavaScript libraries may use the custom data attributes, as they are considered to be part of the page on which they are used. Authors of libraries that are reused by many authors are encouraged to include their name in the attribute names, to reduce the risk of clashes. Where it makes sense, library authors are also encouraged to make the exact name used in the attribute names customizable, so that libraries whose authors unknowingly picked the same name can be used on the same page, and so that multiple versions of a particular library can be used on the same page even when those versions are not mutually compatible.

The dataset IDL attribute provides convenient accessors for all the data-* attributes on an element. On getting, the dataset IDL attribute must return a DOMStringMap whose associated element is this element.