xHTML

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 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.