Work on HTML 5 originally started in late 2003, as a proof of concept to show that it was possible to extend HTML 4'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. 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.

In early 2004, some of the principles that underlie this effort, as well as an early draft proposal covering just forms-related features, were presented to the W3C jointly by Mozilla and Opera at a workshop discussing the future of Web Applications on the Web. The proposal was rejected on the grounds that the proposal conflicted with the previously chosen direction for the Web's evolution.

Shortly thereafter, Apple, Mozilla, and Opera jointly announced their intent to continue working on the effort. A public mailing list was created, and the drafts were moved to the WHATWG site. The copyright was subsequently amended to be jointly owned by all three vendors, and to allow reuse of the specifications.

In 2006, the W3C expressed interest in the specification, and created a working group chartered to work with the WHATWG on the development of the HTML 5 specifications. The working group opened in 2007. Apple, Mozilla, and Opera allowed the W3C to publish the specifications under the W3C copyright, while keeping versions with the less restrictive license on the WHATWG site.

Since then, both groups have been working together.

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 «DOM5 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 «HTML5». This is the format recommended for most authors. It is compatible with most legacy Web browsers. If a document is transmitted with the MIME type text/html, then it will be processed as an «HTML5» document by Web browsers.

The second concrete syntax uses XML, and is known as «XHTML5». When a document is transmitted with an XML MIME type, such as application/xhtml+xml, then it is processed by an XML processor by Web browsers, and treated as an «XHTML5» document. Authors are reminded that the processing for XML and HTML differs; in particular, even minor syntax errors will prevent an XML document from being rendered fully, whereas they would be ignored in the «HTML5» syntax.

The «DOM5 HTML», «HTML5», and «XHTML5» representations cannot all represent the same content. For example, namespaces cannot be represented using «HTML5», but they are supported in «DOM5 HTML» and «XHTML5». Similarly, documents that use the noscript feature can be represented using «HTML5», but cannot be represented with «XHTML5» and «DOM5 HTML». Comments that contain the string «->» can be represented in «DOM5 HTML» but not in «HTML5» and «XHTML5». And so forth.