The differences between HTML and XHTML are small. As the rules of XHTML are based on those of XML, the same exceptionally strict rules apply as with XML. The major differences are outlined below.

The most important difference between the two markup languages is that HyperText Markup Language, or HTML, is an application of SGML (Standard Generalized Markup Language)1 and allows an author to omit certain tags and use attribute minimization. The Extensible HyperText Markup Language, or XHTML, is an application of XML (Extensible Markup Language). It doesn’t permit the omission of any tags or the use of attribute minimization. XHTML documents must start with an XHTML DocType declaration....

The Extensible Hypertext Markup Language, or XHTML, is a hybrid between HTML and XML that is more universally acceptable in Web pages and search engines than XML. Both standards are powerful, but XHTML is significantly better than HTML.

XHTML uses three XML namespaces (used to qualify element and attributes names by associating them with namespaces identified by URI references. Namespaces prevent identically custom-named tags that may be used in different XML documents from being read the same way), which correspond to three HTML 4.0 DTDs: Strict, Transitional, and Frameset. ...