[css-background-4] add background-layers property to set everything but background-color #8726
Description
spec: https://w3c.github.io/csswg-drafts/css-backgrounds/#the-background
We often run into situations where background-color has already been set for an element or component, and in a separate class we want to also add a background image to it.
In order to do this without overriding the color, we have to use the background-image property specifically.
Then we need to size, position, no-repeat it and that's 3 more longhand properties.
Add two bg images and we're managing parallel arrays in every longhand property above to avoid overriding the background-color set elsewhere.
It gets pretty tedious very quickly.
Fortunately, the spec already defines a <bg-layer> so it may not be a big ask to add background-layers as a shorthand property that defines everything for a background, excluding background-color.
background-layers = <bg-layer># | none
(suggesting plural background-layers even though the longhand bg properties are singular since they predate multiple image layers)
Thank you!