View the testcase and hover over the square. It doesn't get animated in Safari (even in the nightlies), it does in Chrome.
AFAICT we're passing the correct transform matrices to Core Animation, and then something goes wrong with the interpolation.
Not a WebKit bug. Kling, can you file this against Core Animations somehow?
reopening and retitling.
How do I reopen? :)
<rdar://problem/11785997>