> Oh? What spec describes it? Can you point me to the specific text that 
> maps HTML markup to a DOM? :-)

A combination of the DTD and the SGML specification, surely.  There are
some issues about ubiquitous non-compliance with the more subtle features
of SGML, but those only really hit people who are determined to serve XHTML
to HTML browsers, and are actually necessary to be able to get away with 
that.

The only ambiguities I'm aware of are in relation to the end of the HEAD
element, and I presume that they are resolved by a longest match heuristic
(as a user, rather than implementor, I can't justify purchasing the actual
SGML specification).

If you are talking about attributes, any issues with poor specification
also apply to XHTML.

(My perception here is that Ann was expressing the false argument for 
XHTML that making all tags non-optional produces a better defined parse
tree; which basically fails to realise that the tags are implied, even
though not physically present.)