XUL is not less standard than QML, MXML or anything of the same sort. It was designed for the same purpose - creating UIs. But it didn't really develop into generally used tool outside Mozilla.

I'm not really sure about what's less webby in QML in comparison to XUL, but the point is, XUL UI already exists now in Firefox in a portable form. Anything outside that will be limiting (unless for example you reimplement the whole thing in QML and rely on Qt portability instead).