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Re: [csswg-drafts] [css-contain-2] content-visibility should pause css animations in subtree (#5611)

From: vmpstr via GitHub <sysbot+gh@w3.org>
Date: Wed, 14 Oct 2020 15:36:31 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-708484259-1602689790-sysbot+gh@w3.org>
> > I don't think option 2 is feasible at all, because it would mean we would have to compute style every time it changes, even while skipped, just in case it affected an animation.
> I'm not sure I understand this point. We would tick the animation's time, but we could leave the elements with dirty style unless it was queried. As far as I can tell, all properties which could start or stop animations are not animatable, i.e. animation-* and display are not animatable properties.

The problem is that when the element is skipped, we don't process its style. This, in turn, means that if script modifies the style in such a way that some element in the subtree now has an animation, then we don't know about this unless we always keep style clean. This is also true if script modifies an element in such a way that, say, increases its animation duration, or adds animation-play-state: paused, or changes any other animation-affecting style.

If we don't have up-to-date style, we can only continue ticking an animation as we knew it at the time we started skipping the element (or, even worse, at the last time style was forced). This is why option 2 seems like a poor fit here.

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