> This *is* true of XML DTDs - if I provide a machine capable of
* understanding XML a DTD and XML, it immediately knows what to do with
* that XML.

XML DTDs have similar power to BNF.  Neither can say what the langauge
actually means, only what is syntactically valid.  XML DTDs cannot even
express all the syntax rules for XHTML, without recourse to human
readable annotations.