[fix] Fixes regression in iFrame animations caused by #6570

[jacwright]

Which is funny, because #6570 was fixing a regression as well, to do with detached dom nodes.

Styles for transitions and animations need to be added to the document in which the element/node resides, otherwise, the CSS won't apply to it and the transition/animation won't run. This was implemented a while ago but broken recently.

Fixes #6637.

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