About
Extensible HTML (XHTML) is an XML document:
- that looks like HTML
- but is well formed according to XML rules.
See also DOM - HTML vs XML document
History
In 1998 1), the W3C membership decided to stop evolving HTML and instead begin work on an XML-based equivalent, called XHTML. This effort started with a reformulation of HTML4 in XML, known as XHTML 1.0, which added no new features except the new serialization, and which was completed in 2000. After XHTML 1.0, the W3C's focus turned to making it easier for other working groups to extend XHTML, under the banner of XHTML Modularization.
The XHTML syntax is an application of XML.
Mime
When a document is transmitted with an XML MIME type, such as application/xhtml+xml, it is treated as an XML document by Web browsers, to be parsed by an XML processor.