Extensible Hypertext Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax. XHTML combines the benefits of XML with those of HTML. For example, to express ideas using new markup, the XHTML family accommodates extensions as described in the XHTML Modularization specification. XHTML also maximizes interoperability: servers, proxies, and user agents can transform content into whatever format is most appropriate for various requesting agents, such as mobile users with limited capabilities, robotic agents that are mainly interested in indexable text, or home users who wish to make the most of broadband or interactive multimedia.

Extensible Markup Language - a simplified subset of the Standard Generalized Markup Language (SGML), meant to be readable by people via semantic constraints; application languages can be implemented in XML. These include XHTML,[4] RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and others. Moreover, XML is sometimes used as the specification language for such application languages.

XML maps are specially formatted links to your pages. They will never replace the need for HTML site maps.