Validating a test
Other tutorial materials fulfill these roles already [Holman],[XPath],[XSLT] ,[Ogbuji C],[Ogbuji U].
During the last few years a number of different XML schema languages have appeared as suggested replacements for the ageing Document Type Definition (DTD).
While XML does include an ID/IDREF mechanism which allows for cross-referencing between elements, and hence another form of relationships, it only weakly binds those elements.
Yet the fundamental approach adopted by these languages does not diverge greatly from the DTD paradigm: the definition of schemas using regular grammars.