As of http://trac.webkit.org/changeset/115989 , the webkit nightlies don't disable overlap testing in RLC for accelerated animations if the animation is confined within an overflow:hidden area. This results in the creation of far fewer layers in some cases. For some reason, the trick doesn't seem to take effect in Chrome. Here are steps to repro: 1. Turn on composited layer borders 2. Navigate to: http://www.flickr.com/ 3. Click on one of the little bullets below the main banner picture to flip to the next image. This activates a CSS transform animation. Notice how in chrome, while the animation is playing there are new layers created below the image. That doesn't happen in WebKit nightlies after 115989 .
I see what the problem is in Chromium. If a layer both animates and clips but has no compositing descendants, then the check in RenderLayerCompositor.cpp:838 fails and the overlap testing isn't put back on. That looks like an oversight to me. My guess is that something other layer gets composited for Safari in that subtree and so that check succeeds, but I haven't verified this.