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Re: [csswg-drafts] [scroll-animations] TAG feedback: interaction with prefers reduced motion (#5321)

From: James Craig via GitHub <sysbot+gh@w3.org>
Date: Thu, 08 Oct 2020 00:20:31 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-705259865-1602116430-sysbot+gh@w3.org>
I mentioned in the call today that I would link to some older discussions about more granular syntax for `@media (prefers-reduced-motion)` in the context of scroll animations. I recalled these as `no-parallax` etc, but the prior-mentioned prefixes were actually `reduce-` and `exclude-`. 

From [December 2016](https://github.com/w3c/csswg-drafts/pull/586#issuecomment-265174560): 

> Future values could be granular tokens, even a combined list, if someone wanted to implement them that way.
> `prefers-reduced-motion: reduce-parallax reduce-scaling; /* reduce-rotation, etc. */`

From [August 2020](https://github.com/w3c/csswg-drafts/issues/2943#issuecomment-672258524):

> iOS is already shipping two variants of a native "reduce motion" setting that differentiates between pans (e.g. automatic side scrolls between screens) and other types of problematic vestibular motion like parallax and scale/zoom. The interface pans often convey a hierarchy. […snip…] 
> One could see a future expansion of prefers-* to one or more optional variants, making the boolean match most useful for authors. 
>`prefers-reduced-motion: no-preference | [ reduce | [exclude-parallax &| exclude-scale &| … ]`

In the context of scroll-animations, please note that [3 of the 5 cited uses cases for prefers-reduced-motion](https://github.com/w3c/csswg-drafts/issues/442#issuecomment-244849793) were for effects that are commonly associated with scroll-jacking animations.

> * animations along a z-axis (e.g. zoom in or zoom out) or scaling of UI elements
> * two or more objects are simultaneously moving in different directions
> * two or more objects are moving at different speeds (parallax or momentum trailing effects)

In 2016, I recall that we decided to forego the granular values until an appropriate use case arose. 

So if the CSS WG is considering a way for authors to mark their scroll-triggered animations as decorative or not, perhaps this is the use case for more discussion on the granularity of `prefers-reduced-motion`.

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