If you try to load html5test.com You will see an impressive score 217, more than the Firefox Desktop which gets 204. There's the support for the WebM and OGG codecs, there is the LocalStorage, Geolocalization.. and other HTML5 stuff, unfortunatly if you try some real world examples It doesn't support that features really.
The font isn't smooth as in webkit and if the page is too long you'll see some gray areas.
Anyway there is the native support to the webfonts and it is powerful enought to run the Fabook site with the Desktop version.
It is very powerful and clean, swiping right you can manage the tabs opened and swiping left you get the bookmark star and the settings.
Anyway a big interface limitation is that the "swipe event" inside the websites, like the HTML5Rocks slides, can't be supported.