IMHO Safari actually has the best hardware acceleration. Chrome-dev channel is catching up quickly.

By hardware acceleration I'm talking about things like -webkit-transform and the support for various 3D transforms. I'm unsure of how it's implemented under the hood but I know Safari kills on 3D transforms and the best part is it works on mobile.

Mere support for CSS transforms is actually really limited. What IE9 and Firefox 4 do on Windows 7 is accelerate all drawing and layer compositing. What it means is that as the browsers gain support for CSS animations and such, they get accelerated for free. Stuff like Canvas and SVG also get accelerated. See http://hacks.mozilla.org/2010/09/hardware-acceleration/.