position-area: add behavioral subfeature for auto margins#29280
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position-area: add behavioral subfeature for auto margins#29280
position-area: add behavioral subfeature for auto margins#29280Conversation
Issue: web-platform-dx/web-features#3558 Issue: w3c/csswg-drafts#10258 (comment) Issue: https://issues.chromium.org/issues/374574260#comment18 Issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1997481 Issue: https://bugzilla.mozilla.org/show_bug.cgi?id=2006398 Issue: https://bugs.webkit.org/show_bug.cgi?id=300823 References: https://developer.apple.com/documentation/safari-release-notes/safari-26_2-release-notes#:~:text=Fixed%20auto%20margins%20by%20converting%20them%20to%20zero%20when%20position%2Darea%20or%20anchor%2Dcenter%20is%20applied%20in%20CSS%20Anchor%20Positioning%2E%20%28162809291
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caugner
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Mar 20, 2026
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Summary
A specification change to anchor positioning now requires that if a non-
noneposition-areaapplies to an element, thenmargin: autoandinset: autoto resolve to 0. This PR captures support for this behavior in a behavior subfeature.Test results and supporting details
Spec
The spec text is this bullet point:
It followed w3c/csswg-drafts#10258 (comment).
Chrome
According to bug 374574260 (comment #18) Chrome appears to have supported this behavior—or something close it—in its initial anchor positioning implementation. Then the spec changed twice, bringing back this original behavior.
Firefox
The actual support for this comes out of Firefox bug 1997481, though the changes to make the test pass came about in Firefox bug 2006398.
Safari
WebKit bug 300823 tracks this; it was fixed by WebKit/WebKit#52419. I determined the version number where it ultimately shipped from the Safari 26.2 Release Notes.
Related issues
position-anchor: capture change in initial value #29272