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.