After implementing Bug 1049975, we support SetEffect for CSSTransition, but we have to make sure it still works if its effect is removed or replaced by a new KeyframeEffect, instead of an ElementPropertyTransition, while this transition is running. According to Bug 1049975 comment 44, we still need to do some work to handle the timing differently if the new transition reverses an existing one .  http://searchfox.org/mozilla-central/rev/740bb4ed16d070cf5d466231b30e80b9204aec54/layout/style/nsTransitionManager.cpp#716-753
This doesn't need to block the Animation interface.
No longer blocks: 1178660
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