HTML is an instance of SGML, a language for specifying markup languages.
After noticing that features of SGML had never been implemented and that there were useful features lcking, people invented called XML.
XHTML is the HTML represented in XML rather than SGML. Many HTML files are not WellFormed XHTML, but can usually be converted relatively simply to XHTML. Those that can't usually have UndefinedSemantics anyway.
9 pages link to XHTML:
