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?
This section is non-normative.
In short: Yes.
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 Standards
overview.
1.3 Background
This section is non-normative.
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 section is non-normative.
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 section is non-normative.
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 (email clients, instant messaging clients, discussion software), document
editing software, etc.
1.6 History
This section is non-normative.
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.
In 2019, the WHATWG and W3C signed an
agreement to collaborate on a single version of HTML going forward: this document.
1.7 Design notes
This section is non-normative.
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
This section is non-normative.
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 globals.
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 Extensibility
This section is non-normative.
HTML has a wide array of extensibility mechanisms that can be used for adding semantics in a
safe manner:
Authors can use the class attribute to extend elements,
effectively creating their own elements, while using the most applicable existing "real" HTML
element, so that browsers and other tools that don't know of the extension can still support it
somewhat well. This is the tack used by microformats, for example.
Authors can include data for inline client-side scripts or server-side site-wide scripts
to process using the data-*="" attributes. These are guaranteed
to never be touched by browsers, and allow scripts to include data on HTML elements that scripts
can then look for and process.
Authors can embed raw data using the <script type="">
mechanism with a custom type, for further handling by inline or server-side scripts.
Authors can extend APIs using the JavaScript prototyping mechanism. This is widely used by
script libraries, for instance.
Authors can use the microdata feature (the itemscope="" and itemprop=""
attributes) to embed nested name-value pairs of data to be shared with other applications and
sites.
Authors can define, share, and use custom elements
to extend the vocabulary of HTML. The requirements of valid custom element names ensure forward compatibility (since no elements will be
added to HTML, SVG, or MathML with hyphen-containing local names in the future).
1.8 HTML vs XML syntax
This section is non-normative.
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.
The in-memory representation is known as "DOM HTML", or "the DOM" for short.
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/htmlMIME 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
This section is non-normative.
This specification is divided into the following major sections:
Documents are built from elements. These elements form a tree using the DOM. This section
defines the features of this DOM, as well as introducing the features common to all elements, and
the concepts used in defining elements.
Each element has a predefined meaning, which is explained in this section. Rules for authors
on how to use the element, along with user agent requirements for how to
handle each element, are also given. This includes large signature features of HTML such
as video playback and subtitles, form controls and form submission, and a 2D graphics API known
as the HTML canvas.
This specification introduces a mechanism for adding machine-readable annotations to
documents, so that tools can extract trees of name-value pairs from the document. This section
describes this mechanism and some algorithms that can be used to convert HTML
documents into other formats. This section also defines some sample Microdata vocabularies
for contact information, calendar events, and licensing works.
HTML documents can provide a number of mechanisms for users to interact with and modify
content, which are described in this section, such as how focus works, and drag-and-drop.
HTML documents do not exist in a vacuum — this section defines many of the features
that affect environments that deal with multiple pages, such as web browsers.
This section describes some mechanisms that applications written in HTML can use to
communicate with other applications from different domains running on the same client. It also
introduces a server-push event stream mechanism known as Server Sent Events or
EventSource, and a two-way full-duplex socket protocol for scripts known as Web
Sockets.
All of these features would be for naught if they couldn't be represented in a serialized
form and sent to other people, and so these sections define the syntaxes of HTML and XML, along with rules for how to parse content using those syntaxes.
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.
For example, "the foo attribute's value must be a valid
integer" is a requirement on producers, as it lays out the allowed values; in contrast,
the requirement "the foo attribute's value must be parsed using the
rules for parsing integers" is a requirement on consumers, as it describes how to
process the content.
Requirements on producers have no bearing whatsoever on consumers.
Continuing the above example, a requirement stating that a particular attribute's value is
constrained to being a valid integer emphatically does not imply anything
about the requirements on consumers. It might be that the consumers are in fact required to treat
the attribute as an opaque string, completely unaffected by whether the value conforms to the
requirements or not. It might be (as in the previous example) that the consumers are required to
parse the value using specific rules that define how invalid (non-numeric in this case) values
are to be processed.
1.9.2 Typographic conventions
This is a definition, requirement, or explanation.
This is a note.
This is an example.
This is an open issue.
This is a warning.
[Exposed=Window]
interfaceExample {
// this is an IDL definition
};
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:
This is a condition
This is another condition
This is the requirement that applies to the conditions above.
This is a third condition
This is the requirement that applies to the third condition.
1.10 A quick introduction to HTML
This section is non-normative.
A basic HTML document looks like this:
<!DOCTYPE html><htmllang="en"><head><title>Sample page</title></head><body><h1>Sample page</h1><p>This is a <ahref="demo.html">simple</a> sample.</p><!-- this is a comment --></body></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:
<p>This is <em>very <strong>wrong</em>!</strong></p>
<p>This <em>is <strong>correct</strong>.</em></p>
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:
<ahref="demo.html">simple</a>
Attributes are placed inside the start tag, and consist
of a name and a value, separated by an "=" character.
The attribute value can remain