Re: [csswg-drafts] [css-anchor-position][other] Handling popover default styles (#10258)

Agenda+ to re-raise @bfgeek's compat concerns in https://github.com/w3c/csswg-drafts/issues/10258#issuecomment-3207602473

In particular, when trying to implement this in WebKit, I found multiple WPT tests were failing because they had set `margin: 0` expecting this to disable the centering behavior, and it seems likely we'll see similar patterns in the wild.

I don't love the idea of disabling `auto` margins in conjunction with `position-area` but it might be the only reasonable option we have given the situation. Here are some options we have:

* If `position-area` is not `none`, disable `auto` margins (unconditionally).
* If `position-area` is not `none` or `center` in an axis, disable `auto` margins.
     * The default alignment in the center track is centering anyway...
* If `position-area` is not `none`, `center`, or `span-all` in an axis, disable `auto` margins.
     * These symmetric cases are the ones where the author is most likely to want to use `auto` margins. If we take this option, we also need to make `anchor-center` disable `auto` margins.
* Do one of the above, but only if alignment is `dialog`.

I think the best option here is the third one, possibly conditioned on `dialog`. I think authors will be confused if they place items into the center track and `auto` margins don't work as they expect.

Also open to other ideas...

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Received on Monday, 6 October 2025 21:59:33 UTC