Accelerated compositing on Windows doesn't work on machines that don't support hardware vertex processing, but should. Safari's Top Sites and Cover Flow features work on these machines, for example.
<rdar://problem/7997692>
Created attachment 56389 [details] Make accelerated compositing work on machines that don't support hardware vertex processing
Might be a dupe of bug 38080.
Committed r59684: <http://trac.webkit.org/changeset/59684>