[web-animations-1] Distinction between document timelines and timelines associated with a document

[cdoublev]

The current text (sectioning and wording) seems to suggest that other types of timelines than document timelines can be associated with a document:

4.3 Timelines

[...]

A timeline can be associated with a document.

4.3.1 Document timelines

A document timeline is a type of timeline associated with a document and whose current time is calculated [...] each time the update animations and send events procedure is run.

But I cannot find any exemple of this.

(1) I would say that any timeline associated with a document should be a document timeline, like the potential subclass mentioned in #5420. If I am correct, I think it would be more appropriate to remove the first paragraph above and move the definition of associated with a document in the next sub-section.

(2) Relatedly, I cannot find where a document timeline is not associated with a document, so I do not see the point of defining associated with a document. If it cannot happen and I am correct in (1), I think it would be clearer to replace associated with a document with document timeline in the procedures checking this condition.

[cdoublev]

(3) I cannot find an exemple of a document timeline that is not the default document timeline. I think it would be useful to clarify the reason of this distinction. My best guess is that this anticipates the need for multiple timelines in the document (like in a video editor).

[birtles]

Yes, the flexibility here is mostly anticipating the SVG use case. I think the simplifications would be ok, however. Regarding (3), if you create a `DocumentTimeline` via the constructor, it is not the default document timeline. Message ID: ***@***.***>

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