[csswg-drafts] Add -webkit- gradients to CSS Images (#3426)

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

== Add -webkit- gradients to CSS Images ==
Currently these are defined at https://compat.spec.whatwg.org/#css-image-type as mapping to specific _dated_ versions of the standardized properties.

It seems better if these are all defined in the same place so we can at least be sure there's a single underlying set of primitives.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/3426 using your GitHub account