[css-conditional] [css-anchor-position] Allow feature detection of anchoring taking transforms into account.

[bramus]

In #8584 (comment) we resolved to have anchor positioning take transforms into account. This change is currently only available in Chrome. I’ve heard from several authors that they would like to feature detect this, as it does have an impact when the UA does or doesn’t do this.

I believe this is a good candidate for @supports named-feature() as I don’t think there is another way how authors would be able to detect this.

[dbaron]

I'm a little puzzled on how to name a feature for this. I'd both like to avoid a name where I have to use positioning rather than position, and I'd like to avoid having to use a word like affects. I don't yet see a good way to combine transform (or transforms) and anchor-position in a way that avoids either of those constraints.

[bramus]

Looking at the proposed name in #13677 (position-sticky-per-axis) a guiding principle here could be to name them $feature-$preposition-$otherthing.

With that in mind, I come to anchor-positioning-with-transforms as a proposed value.

I personally don’t mind anchor-positioning as part of the named feature, as that named is already used in the wild. E.g. https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Anchor_positioning and https://webstatus.dev/features/anchor-positioning.