This illustration shows how the XML family of standards are related.  Both HTML and XML were built on the syntax of SGML.  HTML provides a tag set for Web pages.  CSS provides a stylesheet language  for HTML (and now XML).  CSS is NOT built on SGML syntax.  XML Core standards (such as XML Name and  XML Base) support XML.  XML Schema Definition Language, extends XML 1.0 to provide new ways to constrain XML.

Three classes of standards have been built on the XML syntax. First is RDF and the standards built upon it (in green).  Second are other W3C standards that use XML to specify core Web functionality (in blue). Finally there are over 400 XML application standards developed by W3C, IDEAlliance, OASIS and other standards bodies (in light blue).

The DOM does not use the XML syntax, even though it provides the document object model for both HTML and XML.