[css-backgrounds-4] background-position-x and background-position-y should be interpolable

[graouts]

The current draft of CSS Backgrounds and Borders Module Level 4 defines the background-position-x and background-position-y properties with their "Animation type" set to "discrete" which means these properties cannot be interpolated. That seems like a mistake.

@birtles @stephenmcgruer @flackr

[birtles]

Yes, I notice we seem to animate it as a list of positions in Gecko.

[flackr]

Yes, especially since in css-backgrounds-3 the short-hand background-position is animatable.