Extensible Hypertext Markup Language. A spinoff of the hypertext markup language (HTML) used for creating Web pages. It is based on the HTML 4.0 syntax, but has been modified to follow the guidelines of XML and is sometimes referred to as HTML 5.0.

Prior to HTML5, HTML was considered an application of SGML, a general-purpose markup language framework, whereas XHTML is an application of XML, a more restricted subset of SGML. Unlike HTML, which requires a tolerant HTML-specific parser, XHTML documents are well-formed and so can be parsed by standard XML parsers.

The World Wide Web Consortium (W3C) accepted XHTML 1.0 as a recommendation on January 26, 2000. The World Wide Web Consortium (W3C) accepted XHTML 1.1 as a recommendation on May 31, 2001. The XHTML5 standard is being developed as an XML adaptation of the HTML5 specification.