From #7047, it might be worth looking into the ability to split the declaration of a timeline (together with its scoping) from its actual attachment to a scroll container.

This would allow authors, for example, to declare a name on a subtree and make it available to all descendants of that subtree, and attach it to a scroll container that is a descendant within the subtree. (At the top level, declaring the name on the root element would make it global.)

Maybe something like scroll-timeline-attachment: local | defer | ancestor | closest where:

• local has the current behavior of binding the name to this element’s scroll container
• defer declares and scopes the name, but does not bind it to a scroll container
• ancestor looks up the ancestor chain for a matching timeline name and attaches to that instance; failing a match, declares it locally
• closest looks back up the tree including previous siblings for a matching timeline name (same lookup as animation-timeline), and attaches to the first matching instance; failing a match, declares it locally