Extensible Hypertext Markup Language, or XHTML, is a family of XML markup languages that mirror or extend versions of the widely used Hypertext Markup Language (HTML), the language that web pages are written in. While HTML (prior to HTML 5) was defined as an application of Standard Generalized Markup Language (SGML), a very flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML.

Because XHTML documents need to be well-formed, they can be processed using standard XML tools—unlike HTML, which requires a relatively complex, lenient, and generally custom parser. XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C Recommendation on May 31, 2001. XHTML 5 is undergoing development as of September 2009, as part of the HTML 5 specification. XHTML 1.0 is "a reformulation of the three HTML 4 document types as applications of XML 1.0". resume

The World Wide Web Consortium (W3C) also continues to maintain the HTML 4.01 Recommendation and the specifications for HTML 5 and XHTML 5 are being actively developed. In the current XHTML 1.0 Recommendation document, as published and revised to August 2002, the W3C commented that, "The XHTML family is the next step in the evolution of the Internet. By migrating to XHTML today, content developers can enter the XML world with all of its attendant benefits, while still remaining confident in their content's backward and future compatibility."

However according to a research paper, in 2004, the Web Hypertext Application Technology Working Group (WHATWG) formed, independently of the W3C, to work on advancing ordinary HTML not based on XHTML. Most major browser vendors were unwilling to implement the features in new W3C XHTML drafts, and felt that they didn't serve the needs of modern web development.

The WHATWG eventually began working on a standard that supported both XML and non-XML serializations, HTML 5, in parallel to W3C standards such as XHTML 2. In 2007, the W3C's HTML working group voted to officially recognize HTML 5 and work on it as the next-generated HTML standard.

In 2009, the W3C allowed the XHTML 2 Working Group's charter to expire, acknowledging that HTML 5 would be the sole next-generation HTML standard, including both XML and non-XML serializations.