The fix for bug 187844 needs implementing for the Web Animation code path too.
<rdar://problem/42756351>
*** Bug 189598 has been marked as a duplicate of this bug. ***
So the test behaves as expected with the new engine, there is some flakiness on macOS though, as pointed out by webkit.org/b/189598.
I think the test only passes because of how it's written. I think if the animation hasn't actually started yet the test passes.
Something is off about that test, if I make the animation longer, it fails with the old engine as well.
Forward-duping this to https://bugs.webkit.org/show_bug.cgi?id=204198. *** This bug has been marked as a duplicate of bug 204198 ***