Attached is a test case where the left piechart is div with background: -webkit-canvas().
The right is the source canvas. The canvas is animated using rAF.
I'd expect the left to pic up the changes, but it looks like repainting is not happening.
For example, toggling the background on/off in the element inspector in DevTools,
one sees a few animated frames but then the animation freezes.
Both are animated and working as expected in WebKit nightly (Version 6.0.2 (8536.26.17, 537+))
Google Chrome 25.0.1328.0 (Official Build 168386) canary
OS Mac OS X
WebKit 537.20 (@134994)