[css-variables] Disallow animation-tainted substitutions in any non-animatable property

[andruud]

Currently, if a custom property is animation-tainted, it's not allowed to substitute that custom property into animation- properties, nor the display property (with some conditions).

This means (since it's not mentioned), that it's allowed to substitute an animation-tainted custom property into direction and writing-mode (among others), effectively making those properties animatable. This makes it unnecessarily complicated to specify a reasonable behavior for css-logical properties in animation keyframes, since the physical properties involved change during the animation.

Properties are probably 'not animatable' for a reason, so it seems bad to create a loophole where unwanted complexity can "escape".

I propose that we (unconditionally) disallow animation-tainted custom property substitutions in any property that's 'not animatable'.

For what it's worth, Chrome already has this behavior (possibly since the beginning), and I've seen no complaints.

[andruud]

cc @tabatkins

[tabatkins]

Yes, this seems reasonable to me.

[css-meeting-bot]

The CSS Working Group just discussed [css-variables] Disallow animation-tainted substitutions in any non-animatable property, and agreed to the following:

• RESOLVED: Disallow animation-tainted substitutions in any non-animatable property

The full IRC log of that discussion

<dael> Topic: [css-variables] Disallow animation-tainted substitutions in any non-animatable property
<dael> github: https://github.com//issues/5341
<dael> TabAtkins: andrers pointed out spec disallows putting animation variable into animation property. Reason is circularity and complexity avoidance.
<fantasai> s/??/iank/
<dael> TabAtkins: This does still allow you to put animation tainted into unanimated properties which makes them animatable. Smuggles complexity of animation in. andrers suggests we further restrict to not allow in any not-animatable property
<dael> TabAtkins: Reasonable to me
<dael> Rossen_: Any feedback?
<dael> Rossen_: Objections?
<dael> RESOLVED: Disallow animation-tainted substitutions in any non-animatable property