HTML was created as an application of SGML - the Standard Generalized Markup Language (ISO 8879:1986)
XML is a descendant of SGML, which is easier to implement
XML requires you to:
These make it practical to parse well-formed XML without a priori knowledge of the tags
XHTML uses lower-case for tags and attributes
Old browsers can render XHTML 1.0 provided you follow simple guidelines