`background-*` CSS longhand properties doesn't match layers

[rcj-siteimprove]

Suggestion: It seems that the same thing happens in background: https://drafts.csswg.org/css-backgrounds/#layering but we do not really do that 🤔 (the longhands are not matching layers).
I guess we should fix the background parsing to get the correct thing going. Maybe it is better in a follow-up PR, though.

Originally posted by @Jym77 in #1711 (comment)