CSS3 3D Transform's "transform-style" property, when used with 'flag' value has recently broken on WebKit. You can verify by running the example from Surfin' Safari blog  and noticing the weird white boxes drawn while hovering mouse (or a single tap on mobile mode, as confirmed by Rafael) over the rotating div. I'm going to bisect the code to find the dangling patch.
I've bisected the code and found out that a fix for a patch that avoids redundant state clipping was causing the issue ( http://trac.webkit.org/changeset/144290 ), from bug 111065. Reverting the patch solves the issue, however like Allan commented, it breaks clipping on some Apple HTML5 examples (i.e. http://www.apple.com/html5/showcase/transitions/ ).
Created attachment 191834 [details]
Comment on attachment 191834 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=191834&action=review
> + m_clipStack.apply(m_context3D.get(), false);
This is a boolean trap.
I prefer to have apply(...) and applyIfNeeded(...). From bindDefaultSurface we'd call apply, and from everywhere else applyIfNeeded.
Created attachment 191976 [details]
Added applyIfNeeded() to avoid boolean trap as reviewed by Noam
Created attachment 191980 [details]
ChangeLog typo fixes as reviewed by Allan
Comment on attachment 191980 [details]
Created attachment 191981 [details]
Patch for landing (last minute ChangeLog typo fix)
Comment on attachment 191981 [details]
Clearing flags on attachment: 191981
Committed r145071: <http://trac.webkit.org/changeset/145071>
All reviewed patches have been landed. Closing bug.