[css-sizing-4] Define that stretch sizing treats auto margins as 0 (since sizing wins against margins, while alignment loses). #9902

Author: tabatkins

css-sizing-4/Overview.bs

@@ -1474,8 +1474,9 @@ Stretch-fit Sizing: filling the containing block</h3>
 		Typically produces the same size that would be obtained
 		by the [=self-alignment property=] in that axis
 		being set to ''justify-self/stretch''
-		and all [=sizing properties=] and 'aspect-ratio'
-		being set to their initial values.
+		all [=sizing properties=] and 'aspect-ratio'
+		being set to their initial values,
+		and ''margin/auto'' margins treated as zero.
 
 		Note: In [=multi-line flex container|multi-line=] [=flex layout=],
 		the [=stretch-fit size=] resolves against the [=containing block=]