Re: [csswg-drafts] [CSS2][css-position-3] Negative containing blocks and `auto` margins (#11478)

The Houdini Task Force just discussed ``[CSS2][css-position-3] Negative containing blocks and `auto` margins``, and agreed to the following:

* `RESOLVED: the IMCB size is floored at zero by adjusting weaker inset`
* `RESOLVED: we center in block axis and safe align in inline axis`

<details><summary>The full IRC log of that discussion</summary>
&lt;alisonmaher> TabAtkins: if your CB is negatively sized<br>
&lt;alisonmaher> ...how does it interact with auto margins and sizes<br>
&lt;alisonmaher> ...CSS2 definition is bad and inconsistent implementations<br>
&lt;alisonmaher> ...what do we want?<br>
&lt;alisonmaher> ...CSS2 has diff behavior dependinding on H/V axis. We think that was an accident. Copy and paste error?<br>
&lt;alisonmaher> iank_: the diff is with auto margins in one axis is safe, and other axis is not<br>
&lt;alisonmaher> iank_: required by web compat<br>
&lt;oriol> q+<br>
&lt;alisonmaher> TabAtkins: maybe, but not consistent across browsers<br>
&lt;alisonmaher> oriol: prob web compat. If IMCB is negative ?? if abspos is big enough, then all browser agree<br>
&lt;alisonmaher> ...in the case where subtract index, the IMCB can be negative, servo and blink disagree. blink clamps<br>
&lt;alisonmaher> ...I could align with either. Ian doesn't want it to go negative<br>
&lt;alisonmaher> ...ok with that<br>
&lt;alisonmaher> TabAtkins: cool with clamping sizes to not be negative<br>
&lt;alisonmaher> PROPOSED: the IMCB size is floored at zero by adjusting weaker axis<br>
&lt;fantasai> s/axis/inset<br>
&lt;alisonmaher> iank_: agree, a mess otherwise<br>
&lt;alisonmaher> TabAtkins: objections?<br>
&lt;alisonmaher> RESOLVED: the IMCB size is floored at zero by adjusting weaker inset<br>
&lt;alisonmaher> TabAtkins: if the CB is positive but too small, there is interop<br>
&lt;alisonmaher> oriol: I believe so<br>
&lt;alisonmaher> ....you may have IMCB that is non-negative, but abspos overflows, in one axis you do one thing and another in the other<br>
&lt;alisonmaher> ...I recall an issue when inconsistency between browsers with orthogonal flows but eventually agree<br>
&lt;alisonmaher> fantasai: weaker in block axis?<br>
&lt;alisonmaher> ...shouldn't depend on direction. Should be inline vs block<br>
&lt;fantasai> s/direction/physical axis/<br>
&lt;alisonmaher> TabAtkins: we center in block axis and safe align in inline axis<br>
&lt;alisonmaher> RESOLVED: we center in block axis and safe align in inline axis<br>
&lt;oriol> https://github.com/w3c/csswg-drafts/issues/5374<br>
</details>


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Received on Thursday, 21 August 2025 14:14:06 UTC