[cleverjake]

In no way is this html. its xml. they use xml for the same reason that most do - to define their own elements and behaviors thereof

[josteink]

XML without namespaces, so pretty much just markup.

You know, without namespaces, you take out the eXtensibility-bit.

[pilsetnieks]

> pretty much just markup

SGML, actually. "Just markup" can entail other things, like TeX, for example.