What is xhtml?

Some articles on xhtml:

Comparison Of Layout Engines (XHTML)
... The following tables compare XHTML compatibility and support for a number of layout engines ... Only XHTML 1.1 is considered ... XHTML 1.1 is necessarily XHTML 1.0 Strict with more elements and attributes deprecated ...
Haml
... markup language that is used to describe the XHTML of any web document without the use of traditional inline coding ... However, Haml avoids the need for explicitly coding XHTML into the template, because it is itself a description of the XHTML, with some code to generate dynamic content ...
Access Key
... In XHTML 2, a revised web authoring language, the HTML Working Group of the World Wide Web Consortium deprecated the accesskey attribute in favor of the XHTML Role Access Module ... However, XHTML 2 has been retired in favor of HTML 5, which (as of August 2009) continues to permit accesskeys ...
Haml - Principles
... Markup should be DRY XHTML involves major repetition ... The result is confusing, unreadable XHTML ... XHTML structure should be clear XML and XHTML are formats built upon the idea of a structured document ...
Features - XHTML5
... XML documents must be served with an XML Internet media type such as application/xhtml+xml or application/xml ...