Experience ? I already see this on current web applications - on more serious (i.e. not just a list of pictures/articles) people tend to pixel-push. And since the resolutions don't match, and the HTML5 compliance levels/compatibility of various browsers are bound to be different especially in the early stages of adoption, no way is it going to look okay.

Certainly. BUT. What's preventing website authors doing it NOW ? You can already serve up completely different/adjusted sites with current technologies, and yet, people don't do that, it's more important to fit in the paradigm of the host OS than to adhere to standards - which (while not apparent at first) *wrong* - imagine for example that in Windows you would have to doubleclick URLs to open them or that the site's CSS would get completely overridden by the OS' theme. It's a step back because you bring in OS/app dependency the web fought so hard to get rid of.