To me it looks like speed is one of the major advantages. JavaScript speed is only one thing. There is also render speed. E.g. this runs totally smoothly (with a bit of flickering (z order problems?)) in Chrome: http://keithclark.co.uk/labs/css-fps/nojs/ In Firefox it seems to have something between 1 and 5 fps and is totally glitched out.

JavaScript speed is not what needs to be worked on currently. Render speed is.