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 XHTML 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 XHTML)".

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 XHTML syntax, depending on context.

In the context of the DOM structures, the terms HTML document and XML document are used as defined in the DOM Core specification, and refer specifically to two different modes that Document objects can find themselves in. [DOMCORE] (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.

The term XHTML document is used to refer to both Documents in the XML document mode that contains element nodes in the HTML namespace, and byte streams labeled with an XML MIME type that contain elements from the HTML namespace, depending on context.

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.

When an algorithm B says to return to another algorithm A, it implies that A called B. Upon returning to A, the implementation must continue from where it left off in calling B.

The term "transparent black" refers to the color with red, green, blue, and alpha channels all set to zero.

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 and URI specifications, refer to as a representation is referred to in this specification as a resource. [HTTP] [RFC3986]

The term MIME type is used to refer to what is sometimes called an Internet media type in protocol literature. The term media type in this specification is used to refer to the type of media intended for presentation, as used by the CSS specifications. [RFC2046] [MQ]

A string is a valid MIME type if it matches the media-type rule defined in section 3.7 "Media Types" of RFC 2616. In particular, a valid MIME type may include MIME type parameters. [HTTP]

A string is a valid MIME type with no parameters if it matches the media-type rule defined in section 3.7 "Media Types" of RFC 2616, but does not contain any ";" (U+003B) characters. In other words, if it consists only of a type and subtype, with no MIME Type parameters. [HTTP]

The term HTML MIME type is used to refer to the MIME type text/html.

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.

The term data: URL refers to URLs that use the data: scheme. [RFC2397]

2.1.2 XML

To ease migration from HTML to XHTML, 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", when used in this specification, refers to any element in that namespace, and thus refers to both HTML and XHTML elements.

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, they contain no ":" (U+003A) characters, and their first three characters are not an ASCII case-insensitive match for the string "xml". [XML]

The term XML MIME type is used to refer to the MIME types text/xml, application/xml, and any MIME type whose subtype ends with the four characters "+xml". [RFC3023]

2.1.3 DOM trees

The root element of a Document object is that Document's first element child, if any. If it does not have one then the Document has no root element.

The term root element, when not referring to a Document object's root element, means the furthest ancestor element node of whatever node is being discussed, or the node itself if it has no ancestors. When the node is a part of the document, then the node's root element is indeed the document's root element; however, if the node is not currently part of the document tree, the root element will be an orphaned node.

When an element's root element is the root element of a Document object, it is said to be in a Document. An element is said to have been inserted into a document when its root element changes and is now the document's root element. Analogously, an element is said to have been removed from a document when its root element changes from being the document's root element to being another element.

A node's home subtree is the subtree rooted at that node's root element. When a node is in a Document, its home subtree is that Document's tree.

The Document of a Node (such as an element) is the Document that the Node's ownerDocument IDL attribute returns. When a Node is in a Document then that Document is always the Node's Document, and the Node's ownerDocument IDL attribute thus always returns that Document.

The Document of a content attribute is the Document of the attribute's element.

The term tree order means a pre-order, depth-first traversal of DOM nodes involved (through the parentNode/childNodes relationship).

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 of an attribute value, Text node, or string, means that the length of the text is zero (i.e. not even containing spaces or control characters).

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

In the contexts of events, the terms fire and dispatch are used as defined in the DOM Core specification: firing an event means to create and dispatch it, and dispatching an event means to follow the steps that propagate the event through the tree. The term trusted event is used to refer to events whose isTrusted attribute is initialized to true. [DOMCORE]

2.1.5 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.

A user agent must not consider the types text/plain and application/octet-stream as having a registered plugin.

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]

A plugin can be secured if it honors the semantics of the sandbox attribute.

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.

2.1.6 Character encodings

The preferred MIME name of a character encoding is the name or alias labeled as "preferred MIME name" in the IANA Character Sets registry, if there is one, or the encoding's name, if none of the aliases are so labeled. [IANACHARSET]

An ASCII-compatible character encoding is a single-byte or variable-length encoding in which the bytes 0x09, 0x0A, 0x0C, 0x0D, 0x20 - 0x22, 0x26, 0x27, 0x2C - 0x3F, 0x41 - 0x5A, and 0x61 - 0x7A, ignoring bytes that are the second and later bytes of multibyte sequences, all correspond to single-byte sequences that map to the same Unicode characters as those bytes in ANSI_X3.4-1968 (US-ASCII). [RFC1345]

This includes such encodings as Shift_JIS, HZ-GB-2312, and variants of ISO-2022, even though it is possible in these encodings for bytes like 0x70 to be part of longer sequences that are unrelated to their interpretation as ASCII. It excludes such encodings as UTF-7, UTF-16, GSM03.38, and EBCDIC variants.

The term a UTF-16 encoding refers to any variant of UTF-16: self-describing UTF-16 with a BOM, ambiguous UTF-16 without a BOM, raw UTF-16LE, and raw UTF-16BE. [RFC2781]

The term code unit is used as defined in the Web IDL specification: a 16 bit unsigned integer, the smallest atomic component of a DOMString. (This is a narrower definition than the one used in Unicode.) [WEBIDL]

The term Unicode code point means a Unicode scalar value where possible, and an isolated surrogate code point when not. When a conformance requirement is defined in terms of characters or Unicode code points, a pair of code units consisting of a high surrogate followed by a low surrogate must be treated as the single code point represented by the surrogate pair, but isolated surrogates must each be treated as the single code point with the value of the surrogate. [UNICODE]

In this specification, the term character, when not qualified as Unicode character, is synonymous with the term Unicode code point.

The term Unicode character is used to mean a Unicode scalar value (i.e. any Unicode code point that is not a surrogate code point). [UNICODE]

The code-unit length of a string is the number of code units in that string.

This complexity results from the historical decision to define the DOM API in terms of 16 bit (UTF-16) code units, rather than in terms of Unicode characters.

2.2 Conformance requirements

All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC2119. The key word "OPTIONALLY" in the normative parts of this document is to be interpreted with the same normative meaning as "MAY" and "OPTIONAL". For readability, these words do not appear in all uppercase letters in this specification. [RFC2119]

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

For example, were the spec to say:

To eat a kiwi, the user must:
1. Peel the kiwi.
2. Eat the kiwi flesh.

...it would be equivalent to the following:

To eat a kiwi:
1. The user must peel the kiwi.
2. The user must eat the kiwi flesh.

Here the key word is "must".

The former (imperative) style is generally preferred in this specification for stylistic reasons.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

2.2.1 Conformance classes

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

Conforming HTML5 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.

Web browsers and other interactive user agents

Web browsers that support the XHTML syntax must process elements and attributes from the HTML namespace found in XML documents as described in this specification, so that users can interact with them, unless the semantics of those elements have been overridden by other specifications.

A conforming XHTML processor would, upon finding an XHTML script element in an XML document, execute the script contained in that element. However, if the element is found within a transformation expressed in XSLT (assuming the user agent also supports XSLT), then the processor would instead treat the script element as an opaque element that forms part of the transform.

Web browsers that support the HTML syntax must process documents labeled with an HTML MIME type as described in this specification, so that users can interact with them.

User agents that support scripting must also be conforming implementations of the IDL fragments in this specification, as described in the Web IDL specification. [WEBIDL]

Unless explicitly stated, specifications that override the semantics of HTML elements do not override the requirements on DOM objects representing those elements. For example, the script element in the example above would still implement the HTMLScriptElement interface.

Non-interactive presentation user agents

User agents that process HTML and XHTML documents purely to render non-interactive versions of them must comply to the same conformance criteria as Web browsers, except that they are exempt from requirements regarding user interaction.

Typical examples of non-interactive presentation user agents are printers (static UAs) and overhead displays (dynamic UAs). It is expected that most static non-interactive presentation user agents will also opt to lack scripting support.

A non-interactive but dynamic presentation UA would still execute scripts, allowing forms to be dynamically submitted, and so forth. However, since the concept of "focus" is irrelevant when the user cannot interact with the document, the UA would not need to support any of the focus-related DOM APIs.

Visual user agents that support the suggested default rendering

User agents, whether interactive or not, may be designated (possibly as a user option) as supporting the suggested default rendering defined by this specification.

This is not required. In particular, even user agents that do implement the suggested default rendering are encouraged to offer settings that override this default to improve the experience for the user, e.g. changing the color contrast, using different focus styles, or otherwise making the experience more accessible and usable to the user.

User agents that are designated as supporting the suggested default rendering must, while so designated, implement the rules in the rendering section that that section defines as the behavior that user agents are expected to implement.

User agents with no scripting support

Implementations that do not support scripting (or which have their scripting features disabled entirely) are exempt from supporting the events and DOM interfaces mentioned in this specification. For the parts of this specification that are defined in terms of an events model or in terms of the DOM, such user agents must still act as if events and the DOM were supported.

Scripting can form an integral part of an application. Web browsers that do not support scripting, or that have scripting disabled, might be unable to fully convey the author's intent.

Conformance checkers

Conformance checkers must verify that a document conforms to the applicable conformance criteria described in this specification. Automated conformance checkers are exempt from detecting errors that require interpretation of the author's intent (for example, while a document is non-conforming if the content of a blockquote element is not a quote, conformance checkers running without the input of human judgement do not have to check that blockquote elements only contain quoted material).

Conformance checkers must check that the input document conforms when parsed without a browsing context (meaning that no scripts are run, and that the parser's scripting flag is disabled), and should also check that the input document conforms when parsed with a browsing context in which scripts execute, and that the scripts never cause non-conforming states to occur other than transiently during script execution itself. (This is only a "SHOULD" and not a "MUST" requirement because it has been proven to be impossible. [COMPUTABLE])

The term "HTML5 validator" can be used to refer to a conformance checker that itself conforms to the applicable requirements of this specification.

XML DTDs cannot express all the conformance requirements of this specification. Therefore, a validating XML processor and a DTD cannot constitute a conformance checker. Also, since neither of the two authoring formats defined in this specification are applications of SGML, a validating SGML system cannot constitute a conformance checker either.

To put it another way, there are three types of conformance criteria:

  1. Criteria that can be expressed in a DTD.
  2. Criteria that cannot be expressed by a DTD, but can still be checked by a machine.
  3. Criteria that can only be checked by a human.

A conformance checker must check for the first two. A simple DTD-based validator only checks for the first class of errors and is therefore not a conforming conformance checker according to this specification.

Data mining tools

Applications and tools that process HTML and XHTML documents for reasons other than to either render the documents or check them for conformance should act in accordance with the semantics of the documents that they process.

A tool that generates document outlines but increases the nesting level for each paragraph and does not increase the nesting level for each section would not be conforming.

Authoring tools and markup generators

Authoring tools and markup generators must generate conforming HTML5 documents. Conformance criteria that apply to authors also apply to authoring tools, where appropriate.

Authoring tools are exempt from the strict requirements of using elements only for their specified purpose, but only to the extent that authoring tools are not yet able to determine author intent. However, authoring tools must not automatically misuse elements or encourage their users to do so.

For example, it is not conforming to use an address element for arbitrary contact information; that element can only be used for marking up contact information for the author of the document or section. However, since an authoring tool is likely unable to determine the difference, an authoring tool is exempt from that requirement. This does not mean, though, that authoring tools can use address elements for any block of italics text (for instance); it just means that the authoring tool doesn't have to verify that when the user uses a tool for inserting contact information for a section, that the user really is doing that and not inserting something else instead.

In terms of conformance checking, an editor has to output documents that conform to the same extent that a conformance checker will verify.

When an authoring tool is used to edit a non-conforming document, it may preserve the conformance errors in sections of the document that were not edited during the editing session (i.e. an editing tool is allowed to round-trip erroneous content). However, an authoring tool must not claim that the output is conformant if errors have been so preserved.

Authoring tools are expected to come in two broad varieties: tools that work from structure or semantic data, and tools that work on a What-You-See-Is-What-You-Get media-specific editing basis (WYSIWYG).

The former is the preferred mechanism for tools that author HTML, since the structure in the source information can be used to make informed choices regarding which HTML elements and attributes are most appropriate.

However, WYSIWYG tools are legitimate. WYSIWYG tools should use elements they know are appropriate, and should not use elements that they do not know to be appropriate. This might in certain extreme cases mean limiting the use of flow elements to just a few elements, like div, b, i, and span and making liberal use of the style attribute.

All authoring tools, whether WYSIWYG or not, should make a best effort attempt at enabling users to create well-structured, semantically rich, media-independent content.

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 (referred to as the XHTML syntax), 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.)

2.2.2 Dependencies

This specification relies on several other underlying specifications.


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


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. [DOMCORE]

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

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

  • Attr interface
  • Comment interface
  • DOMImplementation interface
  • Document interface
  • DocumentFragment interface
  • DocumentType interface
  • DOMException interface
  • Element interface
  • Node interface
  • NodeList interface
  • ProcessingInstruction interface
  • Text interface
  • HTMLCollection interface, and the terms collections and represented by the collection
  • DOMTokenList interface
  • DOMSettableTokenList interface
  • createDocument() method
  • createHTMLDocument() method
  • createElement() method
  • createElementNS() method
  • getElementById() method
  • insertBefore() method
  • ownerDocument attribute
  • childNodes attribute
  • localName attribute
  • parentNode attribute
  • namespaceURI attribute
  • tagName attribute
  • id attribute
  • textContent attribute
  • The insert, append, and replace algorithms for nodes
  • Event interface
  • EventTarget interface
  • EventInit dictionary type
  • target attribute
  • isTrusted attribute
  • The type of an event
  • The concept of an event listener and the event listeners associated with an EventTarget
  • 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 a DOM range, and the terms start, end, and boundary point as applied to ranges.
  • MutationObserver interface
  • The MutationObserver scripting environment concept
  • The invoke MutationObserver objects algorithm

The term throw in this specification is used as defined in the DOM Core specification. The following DOMException types are defined in the DOM Core specification: [DOMCORE]

  1. IndexSizeError
  2. HierarchyRequestError
  3. WrongDocumentError
  4. InvalidCharacterError
  5. NoModificationAllowedError
  6. NotFoundError
  7. NotSupportedError
  8. InvalidStateError
  9. SyntaxError
  10. InvalidModificationError
  11. NamespaceError
  12. InvalidAccessError
  13. TypeMismatchError
  14. SecurityError
  15. NetworkError
  16. AbortError
  17. URLMismatchError
  18. QuotaExceededError
  19. TimeoutError
  20. InvalidNodeTypeError
  21. DataCloneError

For example, to throw a TimeoutError exception, a user agent would construct a DOMException object whose type was the string "TimeoutError" (and whose code was the number 23, for legacy reasons) and actually throw that object as an exception.

The URL associated with a Document, as defined in the DOM Core specification, is referred to in this specification as the document's address.

The following features are defined in the DOM Events specification: [DOMEVENTS]

  • MouseEvent interface
  • The MouseEvent interface's screenX attribute
  • The MouseEvent interface's screenY attribute
  • The MouseEvent interface's clientX attribute
  • The MouseEvent interface's clientY attribute
  • The MouseEvent interface's ctrlKey attribute
  • The MouseEvent interface's shiftKey attribute
  • The MouseEvent interface's altKey attribute
  • The MouseEvent interface's metaKey attribute
  • The MouseEvent interface's button attribute
  • The MouseEvent interface's buttons attribute
  • The MouseEvent interface's relatedTarget attribute
  • The UIEvent interface's view attribute
  • The UIEvent interface's detail attribute
  • click 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 the DOM Parsing and Serialization specification: [DOMPARSING]

  • innerHTML
  • outerHTML
  • insertAdjacentHTML

User agents are also encouraged to implement the features described in the HTML Editing APIs and UndoManager and DOM Transaction specifications. [EDITING] [UNDO]

File API

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

  • Blob
  • File
  • FileList

It also uses the following interface defined in the File System API specification: [FILESYSTEMAPI]

  • FileCallback

This specification references the XMLHttpRequest specification to define how the two specifications interact. The terms document response entity body, XMLHttpRequest base URL, XMLHttpRequest origin, and XMLHttpRequest referrer source are defined in that specification. [XHR]


The terms expose, supported property indices, determine the value of an indexed property, support named properties, supported property names, determine the value of a named property, platform array objects, and read only (when applied to arrays) are used as defined in the Web IDL specification. The algorithm to convert a DOMString to a sequence of Unicode characters is similarly that defined in the Web IDL 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 time after that truncation.

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).

Typed Arrays

The ArrayBuffer interface and underlying concepts from the Typed Array Specification are used for several features in this specification. The Uint8ClampedArray interface type is specifically used in the definition of the canvas element's 2D API. [TYPEDARRAY]


Some parts of the language described by this specification only support JavaScript as the underlying scripting language. [ECMA262]

The term "JavaScript" is used to refer to ECMA262, 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 term JavaScript global environment refers to the global environment concept defined in the ECMAScript specification.

Media Queries

Implementations must support the Media Queries language. [MQ]


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

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

  • WebVTT file
  • WebVTT file using cue text
  • WebVTT file using chapter title text
  • WebVTT file using only nested cues
  • WebVTT parser
  • WebVTT cue settings and how to parse the WebVTT settings
  • The rules for updating the display of WebVTT text tracks
  • The WebVTT cue text DOM construction rules
  • The WebVTT cue text rendering rules
  • The WebVTT cue text parsing rules

Implementations must support the semantics of URLs defined in the URI and IRI specifications, as well as the semantics of IDNA domain names defined in the Internationalizing Domain Names in Applications (IDNA) specification. [RFC3986] [RFC3987] [RFC3490]


The following terms are defined in the Cookie specification: [COOKIES]

  • cookie-string
  • receives a set-cookie-string

The following terms are defined in the CORS specification: [CORS]

  • cross-origin request
  • cross-origin request status
  • custom request headers
  • simple cross-origin request
  • redirect steps
  • omit credentials flag
  • resource sharing check
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.

In particular, some features require that a string be parsed as a CSS <color> value. When parsing a CSS value, user agents are required by the CSS specifications to apply some error handling rules. These apply to this specification also. [CSSCOLOR] [CSS]

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.

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

Support for the CSS Object Model is required for implementations that support scripting. The following features and terms are defined in the CSSOM specifications: [CSSOM] [CSSOMVIEW]

  • LinkStyle
  • CSSStyleDeclaration
  • StyleSheet
  • sheet
  • disabled
  • Alternative style sheet sets and the preferred style sheet set
  • Serializing a CSS value
  • Scroll an element into view

Parts of the Fullscreen specification, in particular the top layer concept, are used to define the rendering of the dialog element. [FULLSCREEN]


The following interface is defined in the SVG specification: [SVG]

  • SVGMatrix

This specification does not require support of any particular network protocol, style sheet language, scripting language, or any of the DOM specifications beyond those described 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 the Web Origin Concept specification and the HTTP State Management Mechanism specification (Cookies) as well. [HTTP] [ORIGIN] [COOKIES]

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

2.2.3 Extensibility

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

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.

If such extensions are nonetheless needed, e.g. for experimental purposes, then vendors are strongly urged to use one of the following extension mechanisms:

For markup-level features that can be limited to the XML serialization and need not be supported in the HTML serialization, vendors should use the namespace mechanism to define custom namespaces in which the non-standard elements and attributes are supported.

For markup-level features that are intended for use with the HTML syntax, extensions should be limited to new attributes of the form "x-vendor-feature", where vendor is a short string that identifies the vendor responsible for the extension, and feature is the name of the feature. New element names should not be created. Using attributes for such extensions exclusively allows extensions from multiple vendors to co-exist on the same element, which would not be possible with elements. Using the "x-vendor-feature" form allows extensions to be made without risk of conflicting with future additions to the specification.

For instance, a browser named "FerretBrowser" could use "ferret" as a vendor prefix, while a browser named "Mellblom Browser" could use "mb". If both of these browsers invented extensions that turned elements into scratch-and-sniff areas, an author experimenting with these features could write:

<p>This smells of lemons!
<span x-ferret-smellovision x-ferret-smellcode="LEM01"
      x-mb-outputsmell x-mb-smell="lemon juice"></span></p>

Attribute names beginning with the two characters "x-" are reserved for user agent use and are guaranteed to never be formally added to the HTML language. For flexibility, attributes names containing underscores (the U+005F LOW LINE character) are also reserved for experimental purposes and are guaranteed to never be formally added to the HTML language.

Pages that use such attributes are by definition non-conforming.

For DOM extensions, e.g. new methods and IDL attributes, the new members should be prefixed by vendor-specific strings to prevent clashes with future versions of this specification.

For events, experimental event types should be prefixed with vendor-specific strings.

For example, if a user agent called "Pleasold" were to add an event to indicate when the user is going up in an elevator, it could use the prefix "pleasold" and thus name the event "pleasoldgoingup", possibly with an event handler attribute named "onpleasoldgoingup".

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

For example, while strongly discouraged from doing so, an implementation "Foo Browser" could add a new IDL attribute "fooTypeTime" to a control's DOM interface that returned the time it took the user to select the current value of a control (say). On the other hand, defining a new control that appears in a form's elements array would be in violation of the above requirement, as it would violate the definition of elements given in this specification.

When adding new reflecting IDL attributes corresponding to content attributes of the form "x-vendor-feature", the IDL attribute should be named "vendorFeature" (i.e. the "x" is dropped from the IDL attribute's name).

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.

The conformance terminology for documents depends on the nature of the changes introduced by such applicable specifications, and on the content and intended interpretation of the document. Applicable specifications MAY define new document content (e.g. a foobar element), MAY prohibit certain otherwise conforming content (e.g. prohibit use of <table>s), or MAY change the semantics, DOM mappings, or other processing rules for content defined in this specification. Whether a document is or is not a conforming HTML5 document does not depend on the use of applicable specifications: if the syntax and semantics of a given conforming HTML5 document is unchanged by the use of applicable specification(s), then that document remains a conforming HTML5 document. If the semantics or processing of a given (otherwise conforming) document is changed by use of applicable specification(s), then it is not a conforming HTML5 document. For such cases, the applicable specifications SHOULD define conformance terminology.

As a suggested but not required convention, such specifications might define conformance terminology such as: "Conforming HTML5+XXX document", where XXX is a short name for the applicable specification. (Example: "Conforming HTML5+AutomotiveExtensions document").

a consequence of the rule given above is that certain syntactically correct HTML5 documents may not be conforming HTML5 documents in the presence of applicable specifications. (Example: the applicable specification defines <table> to be a piece of furniture — a document written to that specification and containing a <table> element is NOT a conforming HTML5 document, even if the element happens to be syntactically correct HTML5.)

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.

2.3 Case-sensitivity and string comparison

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

Comparing two strings in an ASCII case-insensitive manner means comparing them exactly, code point for code point, except that the characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) and the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are considered to also match.

Comparing two strings in a compatibility caseless manner means using the Unicode compatibility caseless match operation to compare the two strings. [UNICODE]

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

Converting a string to ASCII uppercase means replacing all characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) with the corresponding characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).

Converting a string to ASCII lowercase means replacing all characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) with the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z).

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.

2.4 UTF-8

When a user agent is required to decode a byte string as UTF-8, with error handling, it means that the byte stream must be converted to a Unicode string by interpreting it as UTF-8, except that any errors must be handled as described in the following list. Bytes in the following list are represented in hexadecimal. [RFC3629]

One byte in the range FE to FF
Overlong forms (e.g. F0 80 80 A0)
One byte in the range C0 to C1, followed by one byte in the range 80 to BF
One byte in the range F0 to F4, followed by three bytes in the range 80 to BF that represent a code point above U+10FFFF
One byte in the range F5 to F7, followed by three bytes in the range 80 to BF
One byte in the range F8 to FB, followed by four bytes in the range 80 to BF
One byte in the range FC to FD, followed by five bytes in the range 80 to BF
One byte in the range C0 to FD that is not followed by a byte in the range 80 to BF
One byte in the range E0 to FD, followed by a byte in the range 80 to BF that is not followed by a byte in the range 80 to BF
One byte in the range F0 to FD, followed by two bytes in the range 80 to BF, the last of which is not followed by a byte in the range 80 to BF
One byte in the range F8 to FD, followed by three bytes in the range 80 to BF, the last of which is not followed by a byte in the range 80 to BF
One byte in the range FC to FD, followed by four bytes in the range 80 to BF, the last of which is not followed by a byte in the range 80 to BF
Any byte sequence that represents a code point in the range U+D800 to U+DFFF
The whole matched sequence must be replaced by a single U+FFFD REPLACEMENT CHARACTER.
One byte in the range 80 to BF not preceded by a byte in the range 80 to FD
One byte in the range 80 to BF preceded by a byte that is part of a complete UTF-8 sequence that does not include this byte
One byte in the range 80 to BF preceded by a byte that is part of a sequence that has been replaced by a U+FFFD REPLACEMENT CHARACTER, either alone or as part of a sequence
Each such byte must be replaced with a U+FFFD REPLACEMENT CHARACTER.

For the purposes of the above requirements, an overlong form in UTF-8 is a sequence that encodes a code point using more bytes than the minimum needed to encode that code point in UTF-8.

For example, the byte string "41 98 BA 42 E2 98 43 E2 98 BA E2 98" would be converted to the string "A��B�C☺�".