The Animation.timeline property of the Animation interface returns or sets the timeline associated with this animation. A timeline is a source of time values for synchronization purposes, and is an AnimationTimeline-based object. By default, the animation's timeline and the Document's timeline are the same.
A timeline object to use as the timing source for the animation, or null to use the default, which is the Document's timeline.
Here we set the animation's timeline to be the same as the document's timeline (this is the default timeline for all animations, by the way):
animation.timeline = document.timeline;AnimationAnimationTimeline the parent object all timelines inherit from.DocumentTimeline the only kind of timeline object available at this time.Document.timeline is the default timeline all animations are assigned.