[csswg-drafts] [css-backgrounds-3] Clarification on animation additive/accmulative composite behavior for background-size (#4614)

shengha2 has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-backgrounds-3] Clarification on animation additive/accmulative composite behavior for background-size ==
From looking at the background-size [spec](https://drafts.csswg.org/css-backgrounds-3/#propdef-background-size), 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. 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/4614 using your GitHub account