[csswg-drafts] [css-anchor-position] What does "using anchor positioning" mean? (#12552)

tabatkins has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-anchor-position] What does "using anchor positioning" mean? ==
We have at least one spot in the spec, currently, where behavior differs based on whether an element is "using anchor positioning" or not (and which isn't directly about anchors) - an element "using anchor positioning" uses the "scrolling containing block" rather than the "local containing block" when a scroll container generates its CB. We are thinking about adding more, like in #10258.

"Using anchor positioning" is currently handwaved as "has a non-`none` value for `position-area`". This is definitely a *sufficient* signal for the use of anchor positioning, but it's not a *necessary* one - an author can use anchor positioning (or at least indicate their interest in anchor positioning) in other ways:

* set `position-anchor` to a non-initial value (but they might be using anchor positioning with the default value)
* use an `anchor()` function in their inset properties
* use an `anchor-size()` function in several of their sizing properties

Talking with @bfgeek, we think we should add at least *some* of these additional signals to the concept (and actually `dfn` the term while we're at it). Which should we add?

1. I think a non-initial `position-anchor` is a gimme. Especially if we do re-add `implicit` as a keyword to be explicit about it (or be more specific about the implicit source, like adding `popover` or `pseudo` or something).
2. I guess use of an anchor function in inset/sizing properties is okay, it's just.... weird. You can "use" anchor functions in ways that don't actually get "used", like variable fallback, and it's unclear how we might want that to trigger. But it's kinda weird if we have behavior designed to make anchorpos work better and it doesn't trigger when you're literally using `top: anchor()`.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/12552 using your GitHub account


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Received on Thursday, 31 July 2025 03:04:34 UTC