Add machinery to assign elements to animators in CSS. Perhaps this also  creates/initializes the animator as we see assignments in CSS. 

The current CSS declaration syntax is: 
animator: <animator-name>
animator-root: <animator-name>


I believe we need also a way to allow CSS animator assignments to happen even before the animator is not yet registered. In another word, it should not an error if we see an animator name in CSS but we still have not received its registration via javascript. This should be valid. We simply upgrade (i.e., animate) those assigned elements once aniamtor is registered.