> It has been argued that the UA should bail out and not render an 
> invalid document, especially as XHTML is an XML application. Couldn't 

That assumes a validating parser.  The main part of the dicsussion 
was about well-formedness.

> this be the case even when DOM manipulation make the construction 
> invalid?

I wouldn't have thought DOM manimpulation could violate well-formedness
as it doesn't work on tags, but rather on the parse tree itself.