Animations for nodes that are not visible are never removed from the list of running animations

[jdm]

<style>
@keyframes boo {
  0%   { opacity: 0; }
  100% { opacity: 1; }
}
#box { animation: boo 1s infinite; }
</style>
<div id="box">hi there!</div>
<script>setTimeout(function() { document.getElementById('box').remove(); }, 3000)</script>

If you look at the list of layout events, the animation continues triggering after the node is removed from the document.

[jdm]

When updating animations as part of regular layout, we should be able to check if there is a fragment associated with the animation's node. If there is not, it is either outside the document or display: none, so we could expire the animation.

[jdm]

We should have a testcase for changing the element's style display to none, as well.

[jdm]

This can happen in recalc_style_for_animations by tracking which animations in animations were actually updated and removing them from the hashmap otherwise.