<Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . In practice, many web development programs provide code validation based on the W3C standards . </P> <P> The root element of an XHTML document must be html, and must contain an xmlns attribute to associate it with the XHTML namespace . The namespace URI for XHTML is http://www.w3.org/1999/xhtml . The example tag below additionally features an xml: lang attribute to identify the document with a natural language: </P> <P> In order to validate an XHTML document, a Document Type Declaration, or DOCTYPE, may be used . A DOCTYPE declares to the browser the Document Type Definition (DTD) to which the document conforms . A Document Type Declaration should be placed before the root element . </P>

Extensible hypertext markup language is not extensible but it is based on xml