[css-syntax] What actually defines handling of url function-token things in property values like background-image?

[bzbarsky]

https://drafts.csswg.org/css-syntax/#consume-ident-like-token defines that url("foo") should return a function token, but I have yet to find a spec that says what to do with

background-image: url("foo");

if a function token is returned... This may not be a syntax issue per se, but it sure does make it impossible to actually implement CSS.

[tabatkins]

https://drafts.csswg.org/css-values/#urls defines it.

[SimonSapin]

https://drafts.csswg.org/css-backgrounds/#image should be changed to link to https://drafts.csswg.org/css-values/#urls