I'm thinking a phone app might factor the interface into multiple panels, while a desktop app may have one composite panel. A pad might not have buttons at all. So what is called "styling" is the hard part - essentially you have to write a new app for each platform.

How cool would it be, if an app specification could be vague, mention the elements by function but the platform determined the instance and layout? Maybe let you set a couple of clues, linking the elements together by properties, letting the app engine choose how to factor the app onto the device.