[css-contain-4] Make content-visibility animatable

[flackr]

css-contain Section 4 currently defines the content visibility property to be Animation type: not animatable. Most likely this was done for simplicity since like display it can affect which animations are running. However, as with display where we are better refining the ways in which it is safe to animate (See #6429), we should figure out in which ways content-visibility can be animated.

In particular, just like display it should be possible for an animation to extend the application of content-visibility: visible in order to allow content to stay visible while an animation is completing. This would be useful for elements like <details> whose contents are content-visibility: hidden when closed.

[chrishtr]

content-visibility was mentioned here, but we just reailzed it wasn't explicitly covered during the CSSWG agenda on the topic. I don't think it will be at all controversial, so propose an async resolution

[flackr]

My simple proposal would be that auto and hidden are both not allowed by owned animations (e.g. CSS animations), and these two values would be treated as revert, as either of these values could result in a CSS animation no longer running. We would also prefer the visible value in interpolation - consistent with our resolution to avoid none in display interpolations.

[astearns]

@flackr can you put a bit more detail into your proposal? I am not entirely sure how to summarize the above for an async resolution.

[flackr]

The three main points are:

1. Make content-visibility animateable
2. Adjust interpolation behavior (similar to display) to avoid the possibly hidden values auto and hidden. Meaning that when interpolating to/from visible the interpolated result for any intermediate progress is visible.
3. 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.

[astearns]

Thanks for explaining further. This seems like a good thing to get in the spec for more review.

The CSSWG will automatically accept this resolution one week from now if no objections are raised here. Anyone can add an emoji to this comment to express support. If you do not support this resolution, please add a new comment.

Proposed Resolution: Make content-visibility animateable as above

[astearns]

RESOLVED: Make content-visibility animateable as above