[csswg-drafts] [css-backgrounds] background-image should be animatable

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

== [css-backgrounds] background-image should be animatable ==
https://drafts.csswg.org/css-backgrounds-3/#the-background-image 
defines background-image to be not animatable (though browsers animate
 some values of it anyway).

`cross-fade()` is defined in 
https://drafts.csswg.org/css-images-3/#cross-fade-function which can 
be used to interpolate between different image values. It would make 
sense to define animation of background-image in terms of this 
function.

Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/578 using your GitHub 
account