XHTML is the abbreviation for the Extensible Hypertext Markup Language. It is a markup language, which has the same depth of expression as HTML but also conventional to XML syntax.      

It is very important to have XHTML Validation as it helps to assure reliability of a website. An XHTML document that conforms to an XHTML specification is said to be valid. Validity assures consistency in document code, which in turn eases processing, but does not necessarily ensure consistent rendering by browsers.

A document can be checked for validity with the W3C Markup Validation Service, governed by the World Wide Web Consortium. In practice, many web development programs provide code validation based on the W3C standards. W3C allows Internet users to check HTML documents for conformance to HTML or XHTML standards.