I agree that the spec definition of this function isn't super clear
about that, but from "Web Animations 1 - 4.5. Animations"[1]:
An animation is a timing node that binds an animation effect child,
called its associated effect, to a timeline parent so that it runs. Both
of these associations are optional and configurable such that an
animation can have no associated effect or timeline at a given moment.
[1]: https://drafts.csswg.org/web-animations-1/#animations
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