...but feel compelled to point out that Moz and IE can validate an XML document against an XML DTD or schema. I'd lay odds that you could achieve some degree of relational testing with that. More than that, though, and you need Cleverness that might cause Pain to roll out.

Of course, client-side RDBMS systems bring their own questions of data synchronisation and transfer that I'll gladly defer to your good self.

Personally, I'd be inclined toward the solution that delivers XHTML 1.0 and nothing else to the desktop, but that's just me.

[Note. Googling for XML and databases is frustrating! All you get is how all the old tools can now emit/talk XML!]