XML also has a LONG history of being used for UI in stuff like QT, which is great.
The one problem I see is that as developers, we shouldn't give a rat's ass about the otuput format's readability when compared to the API ergonomics. If the API for describing the UI is good enough (Can I do stuff like "App.Transition(App.findOne(Sidebar), 500, {width:"1%"}, {width:"30%"}, Ease.EASE_IN_OUT)" or is it a 300loc tutorial?) then the storage format at the end of the day is not THAT important, at least for me, a guy who does a lot of GUI work every day.