This page http://dbaron.org/css/test/2009/transitions-alpha shows a background pattern shift when the color transitions run.
This is a very bad case of bug 26747. *** This bug has been marked as a duplicate of bug 26747 ***
Reopening. This does not appear to be a duplicate of bug 26747 after all. I think it may be a Core Graphics issue, but that requires a little further investigation.
<rdar://problem/7065684>
This is indeed not a WebKit bug. Resolving as invalid.