[css-position-3] Clarify interaction of auto margins and stretch alignment sizing #11902

Author: fantasai

css-position-3/Overview.bs

@@ -909,19 +909,19 @@ Absolute Positioning Layout Model</h2>
 <h3 id="abspos-auto-size">
 Automatic Sizes of Absolutely-Positioned Boxes</h3>
 
-	The [=automatic size=] of an [=absolutely positioned box=] is resolved against its [=inset-modified containing block=] as follows
-	(treating ''margin/auto'' [=margins=] as zero):
+	The [=automatic size=] of an [=absolutely positioned box=]
+	is resolved against its [=inset-modified containing block=] as follows:
 
 	<dl class=switch>
 		: If its [=self-alignment property=] in the relevant axis is ''align-self/stretch''
+			and neither of its [=inset properties=] nor [=margins=] in that axis are ''margin/auto''
 		: Or if it is ''align-self/normal''
 			and the box is [=non-replaced=],
 			not a [=table wrapper box=],
 			and has no ''inset/auto'' [=inset property|inset=] in the relevant axis
 		::
 			Its [=automatic size=] is its [=stretch-fit size=].
 
-
 		: Otherwise
 		::
 			Its [=automatic size=] is its [=fit-content size=].
@@ -937,6 +937,9 @@ Automatic Sizes of Absolutely-Positioned Boxes</h3>
 	An [=automatic size=] in the [=ratio-determining axis=]
 	is determined as above.
 
+	For the purpose of computing these sizes,
+	any ''margin/auto'' [=margins=] are treated as zero.
+
 	The [=automatic minimum size=] of an absolutely-positioned box is always zero.
 
 	Note: To the extent that form controls can be resized