The API for WinRT is fully baked since it's a sandboxed API, but the native code that browsers need is still somewhat in flux I think.

Also, that's for x86 only. ARM devices are nowhere as polished. In fact they seem to have specifically made them unavailable and locked them up in glass boxes at BUILD and also didn't allow reporters to play with them at MWC.

http://www.engadget.com/2011/09/14/nvidias-kal-el-reference-...

Maybe they're concentrating on Metro apps first since the browser developers have significant resources to ramp up development unlike the small app developers, I am not sure though.