[css-backgrounds] background-size should be animatable if keywords match

[alancutter]

By my reading of the background-size spec, values that use keywords are not animatable.

https://drafts.csswg.org/css-backgrounds/#the-background-size

Animatable: as repeatable list of simple list of length, percentage, or calc (This means keyword values are not animatable.)

This means that you can't animate from background-size: 10px to background-size: 20px due to the implicit presence of the auto keyword (background-size: 10px computes to background-size: 10px auto).

Chrome <=50, Firefox, Safari and Edge will animate the above example, IE doesn't animate background-size.
Chrome 51 stopped animating these cases and received a bug report about it: http://crbug.com/616072
Chrome intends to revert its behaviour for compatibility with other browsers and animate background-size when keywords are positionally matched.

[tabatkins]

Nope, keywords are animatable. Check the computed value definition carefully - all values get converted into a pair of length-percentages.

[shans]

"Computed value: as specified, but with lengths made absolute and omitted ‘auto’ keywords filled in"

I read that as saying that 'background-size: 10px' computes to 'background-size: 10px auto'. Is that an incorrect reading?

[alancutter]

I should be clearer with my example. I propose that background-size: 50%, cover and background-size: 100%, cover should be able to smoothly interpolate since the auto and cover keywords are matched.

[tabatkins]

So hey, I'm a dummy, I was thinking of background-position.

@alancutter That example is way more understandable, and I completely agree it should be animatable.