1. 2 Common infrastructure
    1. 2.1 Terminology
      1. 2.1.1 Resources
      2. 2.1.2 XML compatibility
      3. 2.1.3 DOM trees
      4. 2.1.4 Scripting
      5. 2.1.5 Plugins
      6. 2.1.6 Character encodings
      7. 2.1.7 Dependencies
      8. 2.1.8 Extensibility
    2. 2.2 Policy-controlled features

2 Common infrastructure

This specification depends on Infra. [INFRA]

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 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.

2.1.2 XML compatibility

To ease migration from HTML to XML, user agents 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]

2.1.3 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 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).

An HTML element can have specific HTML element insertion steps defined for the element's local name. Similarly, an HTML element can have specific HTML element removing steps defined for the element's local name.

The insertion steps for the HTML Standard, given insertedNode, are defined as the following:

  1. If insertedNode is an element whose namespace is the HTML namespace, and this standard defines HTML element insertion steps for insertedNode's local name, then run the corresponding HTML element insertion steps given insertedNode.

  2. If insertedNode is a form-associated element or the ancestor of a form-associated element, then:

    1. If the form-associated element's parser inserted flag is set, then return.

    2. Reset the form owner of the form-associated element.

The removing steps for the HTML Standard, given removedNode and oldParent, are defined as the following:

  1. Let document be removedNode's node document.

  2. If document's focused area is removedNode, then set document's focused area to document's viewport, and set document's relevant global object's navigation API's focus changed during ongoing navigation to false.

    This does not perform the unfocusing steps, focusing steps, or focus update steps, and thus no blur or change events are fired.

  3. If removedNode is an element whose namespace is the HTML namespace, and this standard defines HTML element removing steps for removedNode's local name, then run the corresponding HTML element removing steps given removedNode and oldParent.

  4. If removedNode is a form-associated element or the ancestor of a form-associated element, then:

    1. If the form-associated element has a form owner and the form-associated element and its form owner are no longer in the same tree, then reset the form owner of the form-associated element.

  5. If removedNode's popover attribute is not in the no popover state, then run the hide popover algorithm given removedNode, false, false, and false.

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.

A node is browsing-context connected when it is connected and its shadow-including root's browsing context is non-null. A node becomes browsing-context connected when the insertion steps are invoked with it as the argument and it is now browsing-context connected. A node becomes browsing-context disconnected either when the removing steps are invoked with it as the argument and it is now no longer browsing-context connected, or when its shadow-including root's browsing context becomes null.

2.1.4 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 operate on the actual underlying data, not a snapshot of the data.

2.1.5 Plugins

The term plugin refers to an implementation-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 navigables 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 navigable 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]

2.1.6 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]

2.1.7 Dependencies

This specification relies on several other underlying specifications.

Infra

The following terms are defined in Infra: [INFRA]

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
  • The BOM sniff algorithm which takes a byte stream and returns an encoding or null.
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 name 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]

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

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

HTTP and related specifications

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

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

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

The following terms are defined in Structured Field Values for HTTP: [STRUCTURED-FIELDS]

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

Fetch

The following terms are defined in Fetch: [FETCH]

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

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

The following terms are defined in Subresource Integrity: [SRI]

Paint Timing

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

Navigation Timing

The following terms are defined in Navigation Timing: [NAVIGATIONTIMING]

Resource Timing

The following terms are defined in Resource Timing: [RESOURCETIMING]

Performance Timeline

The following terms are defined in Performance Timeline: [PERFORMANCETIMELINE]

Long Tasks

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

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:

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.

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

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

User agents that support JavaScript must also implement the Import Attributes proposal. The following terms are defined there, and used in this specification: [JSIMPORTATTRIBUTES]

User agents that support JavaScript must also implement the JSON modules proposal. The following terms are defined there, and used in this specification: [JSJSONMODULES]

User agents that support JavaScript must also implement the Resizable ArrayBuffer and growable SharedArrayBuffer proposal. The following terms are defined there, and used in this specification: [JSRESIZABLEBUFFERS]

WebAssembly

The following term is defined in WebAssembly JavaScript Interface: [WASMJS]

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]

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

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

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

The following events are defined in Clipboard API and events: [CLIPBOARD-APIS]

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 following features are defined in Selection API: [SELECTION]

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

The following features are defined in Fullscreen API: [FULLSCREEN]

High Resolution Time provides the current high resolution time, the relative high resolution time, the unsafe shared current time, the shared monotonic clock, the coarsen time algorithm, and the DOMHighResTimeStamp typedef. [HRT]

File API

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

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]

Media Capture and Streams

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

Reporting

The following terms are defined in Reporting: [REPORTING]

XMLHttpRequest

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

Battery Status

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

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:

  1. Let color be the result of parsing input as a CSS <color>. [CSSCOLOR]

  2. If color is failure, then return failure.

  3. 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.

  4. Return color.

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

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 Box Model: [CSSBOX]

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

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

The following terms are defined in CSS Images: [CSSIMAGES]

The term paint source is used as defined in CSS Images Level 4 to define the interaction of certain HTML elements with the CSS 'element()' function. [CSSIMAGES4]

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

CSS Backgrounds and Borders also defines the following border properties: [CSSBG]

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

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

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

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

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

The following terms are defined in CSS Inline Layout: [CSSINLINE]

The following terms and features are defined in CSS Box Sizing: [CSSSIZING]

The following features are defined in CSS Lists and Counters. [CSSLISTS]

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

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

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

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 following features are defined in CSS Text: [CSSTEXT]

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

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

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]

The following features are defined in CSS View Transitions: [CSSVIEWTRANSITIONS]

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]

The following terms are defined in the CSS Scoping: [CSSSCOPING]

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

The following term is defined in CSS Pseudo-Elements: [CSSPSEUDO]

The following terms are defined in CSS Containment: [CSSCONTAIN]

Intersection Observer

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

Resize Observer

The following terms are defined in Resize Observer: [RESIZEOBSERVER]

WebGL

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

WebGPU

The following interfaces are defined in WebGPU: [WEBGPU]

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:

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]

Content Security Policy

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

Service Workers

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

Secure Contexts

The following algorithms are defined in Secure Contexts: [SECURE-CONTEXTS]

Permissions Policy

The following terms are defined in Permissions Policy: [PERMISSIONSPOLICY]

Payment Request API

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

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]

Filter Effects

The following features are defined in Filter Effects: [FILTERS]

Compositing

The following features are defined in Compositing and Blending: [COMPOSITE]

Cooperative Scheduling of Background Tasks

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

Screen Orientation

The following terms are defined in Screen Orientation: [SCREENORIENTATION]

Storage

The following terms are defined in Storage: [STORAGE]

Web App Manifest

The following features are defined in Web App Manifest: [MANIFEST]

WebCodecs

The following features are defined in WebCodecs: [WEBCODECS]

WebDriver

The following terms are defined in WebDriver: [WEBDRIVER]

WebDriver BiDi

The following terms are defined in WebDriver BiDi: [WEBDRIVERBIDI]

Web Cryptography API

The following terms are defined in Web Cryptography API: [WEBCRYPTO]

WebSockets

The following terms are defined in WebSockets: [WEBSOCKETS]

WebTransport

The following terms are defined in WebTransport: [WEBTRANSPORT]

Web Authentication: An API for accessing Public Key Credentials

The following terms are defined in Web Authentication: An API for accessing Public Key Credentials: [WEBAUTHN]

Credential Management

The following terms are defined in Credential Management: [CREDMAN]

Console

The following terms are defined in Console: [CONSOLE]

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.

2.1.8 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.

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.

2.2 Policy-controlled features

Headers/Permissions-Policy/document-domain

Support in one engine only.

Firefox🔰 74+SafariNoChrome🔰 88+
Opera?Edge🔰 88+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome AndroidNoWebView Android?Samsung Internet?Opera Android?

This document defines the following policy-controlled features:

Headers/Feature-Policy/autoplay

Firefox🔰 74+SafariNoChrome64+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Headers/Permissions-Policy/autoplay

Support in one engine only.

Firefox🔰 74+SafariNoChrome88+
Opera?Edge88+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?