@chrisnz: No comment, on my vague comment, as I don't speak for Microsoft. Plus I don't really know what's going on either. Mobile is doubling every 12 months, but desktop browsing is going away (still 95%). Hedge your bets and do both, with mobile-first design. And add incremental features when you "detect" desktop browsers.
@Zwolf: The decision to use a pure plug-in approach, like Silverlight, vs. straight HTML5, is really dependent on your particular situation with your customer. If IP obfuscation is important, then Silverlight is great. Simple testing path? Silverlight. SEO and text-based content or forms? HTML5.
There's always a hybrid approach too. Think of it as <silverlight>. Check for it, and have a fall-back plan. Or beg the user to install Silverlight. It's there about 65% of the time.
Rui: covered! In fact, it's one of the key tenents of HTML and "flexible grids". Your site should at least be functional on mobile, and work well with touch-enabled devices. And buttons need to be 40x40 for fat fingers, like mine.