As of today's mozilla-central [1], before-change style is the value that has been already updated at the *current time* if the target element has already animations there. That's because we are doing cascading process in animation-only restyle (bug 1361938). To get the correct before-change style, we have to store rule nodes at the previous style change event somewhere and cascade this previous rule nodes with animation rules nodes which are updated at the current time. I have no idea how to do this effectively. [1] https://hg.mozilla.org/mozilla-central/rev/4541134e973a6bd5e667a603e844854c8e5361da

If the current patches in bug 1361938 gets landed as it is, this bug will be a duplicated.

Fixed in bug 1361938.