But what would that CSS end up looking like? There will be a lot of proprietary extensions (at least their should be) and a lot of quixotic classes and element id's that you'd probably have to work around. I envision something like Office's awful "open" format - the same people are driving this bus.

I've done the MS HTML/JS client route for a long time - I've set up a lot of gadgets, and InfoPath(!) has an HTML/XML/JS programming model. Truth be told this model is actually really painful to work with if all you have is HTML/JS. Keep in mind that the "web" isn't just HTML. It's Perl, Rails, Json, JS, Google, Wikipedia, RSS, etc.

Now, if these tiles are something akin to mini-web browsers, where you can use the full power of the web & web technologies (is this what they are actually saying?), that is a different story.