[web-animations] Tighten up some examples

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extra : rebase_source : 4dc87a00071df4626361a9eb50eb7bc6f4a60dab
[web-animations] Clarified when effect callbacks are called

Based on source prepared by Douglas Stockwell:

https://docs.google.com/document/d/14JXp8KXOexVC7t5ogVOQBvhZBH3cK4YRiuV1sxteBk4/edit

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extra : rebase_source : 5634e3fdd371d82b72922dbd101cfb4a1ecf7c69