I think the interesting part about XUL and why I really enjoyed it back in the day is that it's what HTML-for-applications could have been. It's still a "document" in the way any XML is a "document", but the markup is way more application-focused than document-focused. But instead we keep using a markup language that's designed for documents and try our best to make it usable in a very not-a-document way.

Edit: one detail I don't recall is what XUL actually uses for rendering components on-screen.