WebCore lacks logic where knowing if elements up front obscure stuff behind it. Right now it just repaints everything behind it from back-to-front.
SVG compositing? Currently doesn't use RenderLayer at all. Probably need to split up RenderLayer with a separate compositing piece which SVG could use.
GPU Accelerated Compositing in Chrome
updated May 2012
This document provides background and details on the implementation of hardware-accelerated compositing in Chrome.
Interview Question On Webkit/Browser