XML for linguists?

I'm wondering if there is or should be some kind of
XML definition for language parsing.

http://www.w3.org/XML/

Anybody know anything about this?
What I vaguely have in mind is something like:

<sentence>
<np case=subject>
...
</np>
<vp voice=antiantiantipassive>
...
</vp>
</sentence>