[css-grid-2] Animation / interpolate grid-auto-columns and grid-auto-rows

[flackr]

In #3201 there were specific animation type rules added for grid-template-columns and grid-template-rows. In https://crbug.com/1365967 @bramus raised a use case where it would be nice to interpolate grid-auto-columns and grid-auto-rows.

It doesn't look like any browser supports this today, based on my simple demo but I don't see why we shouldn't support interpolating the auto properties as well. Given the syntax is only <track-size>+ I think the animation type may need to be:

  Animation type: if the list lengths match, by computed value type per item; discrete otherwise

Unless per item animation is already implied by the animation type of a list of track sizes?

@fantasai

[tabatkins]

It's already marked as animatable, and lists should default to animating in that fashion. But we went ahead and clarified this based on your comment. ^_^