[css-contain-3] Make content-visibility animatable.

[flackr]

Allow animating content-visibility as animation type discrete with special interpolation to avoid hidden values similar to visibility. Fixes #8627

[mirisuzanne]

This generally makes sense to me. The linked resolution had a third aspect, and it's not clear to me if it needs to be captured here:

Follow the same pattern as display for ensuring that css animations cannot deactivate themselves. Currently following that pattern would mean treating auto and hidden as revert.

[flackr]

We're shifting away from that requirement for display as fewer special cases is simpler. See #6429 (comment)

Also I realized after making the recommendation for content-visibility that the same logic does not apply. content-visibility affects the visibility of its content, but the element itself remains visible (runs animations) even if set to hidden.

[mirisuzanne]

Want to add a line in the changelog about this?

[flackr]

Want to add a line in the changelog about this?

Thanks for the reminder, done.