Right, this is an interesting topic, and it does really depend on the browser + graphics card. I've been meaning to do some research on this for a while, so here's a quick summary.
Webkit (Safari, Chrome, iOS, Android)
In Webkit, AnimationBase.cpp used to specify it, but it seems to have moved! Doh! Well, anyway, if you search the code for ACCELERATED_COMPOSITING, you'll find it.
Specifically, if something matches one of these, then it can be accelerated (at least in Chromium):
- Layer has 3D or perspective transform CSS properties
- Layer is used by element using accelerated video decoding
- Layer is used by a element with a 3D context or accelerated 2D context
- Layer is used for a composited plugin
- Layer uses a CSS animation for its opacity or uses an animated webkit transform
- Layer uses accelerated CSS filters
- Layer has a descendant that is a compositing layer
- Layer has a sibling with a lower z-index which has a compositing layer (in other words the layer is rendered on top of a composited layer)
Source
Gecko (Firefox)
In Gecko, https://wiki.mozilla.org/Platform/GFX/HardwareAcceleration explains their plan/implementation, much like IE, on XP the options are more limited, but every other OS gets some acceleration if the graphics card is supported properly. Firefox 4 (I think!) added acceleration for text, canvas and transforms.
Trident (IE)
IE10 seems to have pretty much everything HW accelerated. http://blogs.msdn.com/b/ie/archive/2011/04/26/understanding-differences-in-hardware-acceleration-through-paintball.aspx
It is pretty quick, so I don't really doubt their claim!
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