XHTML is a reformulation of
HTML in
XML.
HTML is based on
SGML and uses some features of
SGML which are not available in
XML (most notably, markup minimization), and consequently
HTML documents typically are not valid
XML documents. XHTML redefines
HTML as an
XML DTD, and also gives some recommendations how to use the markup in order to make XHTML compatible with older browsers, which only understand
HTML (and thus may have problems processing
XML syntax, for example the special form of empty elements).