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.
The difference though is if the intent is for the markup to be styled, it will be written differently.
For example, take a look at the WPF built-in controls. Those are written in very clean XAML. Easy to read, style, and modify. I'm sure they could have made it difficult, but their intent was that you'd modify the built-in controls.
If they want to do this with the UI, they could do it. The theming you could do with this could be incredible.
I doubt this is something they're going to do for V1 (if ever), but I can dream.
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.