[css-backgrounds-3] Clarification on animation additive/accmulative composite behavior for background-size

[shengha2]

From looking at the background-size spec, there seems to have no additive/accumulative behavior defined. The 'list' type is not a css-value type. I think there might be some unspec area here. cc @birtles

Some background for what I am doing: I am porting some Chrome specific composition tests to external and found some tests might have some unspec behavior during composition.

[Loirooriol]

The meaning of "Animation type: repeatable list" is defined in https://drafts.csswg.org/web-animations-1/#repeatable-list

[fantasai]

Marking as invalid per #4614 (comment) @shengha2 Let me know if this is incorrect.