If a DOM::Element has an animation-name property, then in addition to
remembering where it came from, it will also remember the
Animations::Animation object that was created for it. This allows
StyleComputer to cancel that animation if the animation-name property
changes as well as to apply any changes required (for example, if
animation-play-state changes from "running" to "paused", it needs to
call .pause() on the animation).