The current RenderLayerCompositor algorithm only makes use of painting order trees when deciding which layers should be composited. This means that an element like <video> can kick a bunch of non-overlapping elements into compositing layers, with resulting impact on text antialiasing. We can improve the RenderLayerCompositor algorithm to look at overlap.
<rdar://problem/7011947>
Created attachment 32362 [details] Patch, changelog
Comment on attachment 32362 [details] Patch, changelog r=me
http://trac.webkit.org/changeset/45598