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 in which web pages are written.

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.