Replying to myself (hey, I like myself and I'd like to have a conversation with me):

CSS doesn't have it right? Just Chrome.

Considering how all kinds of "experts" have started to make web sites that only work fine in Chrome [1], this is not exactly a useful new feature, more like embrace and extend...

[1] Orange Romania, when will I be able to download my invoices again in Firefox?

It's in the draft spec for the CSS Values and Units Module Level 5 (https://drafts.csswg.org/css-values-5/#if-notation)

I don't see Mozilla's or WebKit's positions anywhere, so this is a Chromium-only feature for now.

Curiously, Can I Use says:

> All major browser engines are working on implementing this spec.

However their official positions are absent:

https://github.com/mozilla/standards-positions/issues/1167

https://github.com/WebKit/standards-positions/issues/453

And Firefox’s bug for implementing it has no activity:

https://bugzilla.mozilla.org/show_bug.cgi?id=1981485

And Can I Use’s underlying data doesn’t have this note:

https://github.com/Fyrd/caniuse/blob/fedfb067aceccb2a5edadcc...

So it’s unclear why the Can I Use website is saying that everybody is working on it.

Maybe the site is "AI" generated now.