XHTML (Extensible HyperText Markup Language) 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 HTML5) 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 parsed
using standard XML parsers—unlike HTML, which requires a lenient HTML-
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.
XHTML5 is undergoing development as of September 2009, as part of the
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