This is rare and hard to reproduce, but if you go into pause state when in the AnimationStateStartWaitStyleAvailable, the state of m_pauseTime is incorrect, leading to an assert when coming out of pause state.
It's actually easy to reproduce, but you have to use the "pause API" (a feature available in LayoutTests) to get it to happen. You simply run a debug build, start a transition and then pause and unpause. The assert is usually hit. I will try to post a test.
Created attachment 62236 [details]
Comment on attachment 62236 [details]
> Index: WebCore/ChangeLog
> + https://bugs.webkit.org/show_bug.cgi?id=37993
The bug title should be on the line above the url ("webkit-patch upload" will do this for you).
Patch landed in http://trac.webkit.org/changeset/63862
After this patch animations/play-state.html is flakey on Qt bot:
Could you check what the problem is?