[css-grid-2] Sync auto margins changes from css-grid-1. w3c#5923

Author: fantasai

css-grid-1/Overview.bs

@@ -4697,7 +4697,7 @@ Changes since the <a href="https://www.w3.org/TR/2020/CRD-css-grid-1-20201218/">
 			now that [=non-replaced elements=] can also have an aspect ratio
 			via 'aspect-ratio'. [[CSS-SIZING-4]]
 			(<a href="https://github.com/w3c/csswg-drafts/issues/6069">Issue 6069</a>)
-		<li id="change-2020-autm-margins">
+		<li id="change-2020-auto-margins">
 			Made [[#auto-margins]] normative and tightened up wording,
 			since some aspects of this behavior were not otherwise defined elsewhere.
 			(part of <a href="https://github.com/w3c/csswg-drafts/issues/5923">Issue 5923</a>)

css-grid-2/Overview.bs

@@ -4086,21 +4086,24 @@ Gutters: the 'row-gap', 'column-gap', and 'gap' properties</h3>
 <h3 id='auto-margins'>
 Aligning with <a value for="margin">auto</a> margins</h3>
 
-	<em>This section is non-normative.
-		The normative definition of how margins affect grid items is in [[#layout-algorithm]].</em>
-
-	Auto margins on <a>grid items</a> have an effect very similar to auto margins in block flow:
+	Auto [=margins=] on <a>grid items</a> have an effect
+	very similar to auto margins in [=block layout=]:
 
 	<ul>
 		<li>
-			During calculations of <a>grid track</a> sizes, auto margins are treated as ''0''.
+			During calculations of <a>grid track</a> sizes,
+			''margin/auto'' margins are treated as ''0''.
 
 		<li>
-			''margin/auto'' margins absorb positive free space
-			prior to alignment via the <a>box alignment properties</a>.
+			As defined for the [=inline axis=] of [=block layout=]
+			(see <a href="https://www.w3.org/TR/CSS2/visudet.html#blockwidth">CSS2&sect;10.3.3</a>),
+			''margin/auto'' margins in either axis absorb positive free space
+			prior to alignment via the <a>box alignment properties</a>,
+			thereby disabling the effects of any [=self-alignment properties=]
+			in that axis.
 
 		<li>
-			Overflowing elements ignore their ''margin/auto'' margins
+			Overflowing [=grid items=] resolve their ''margin/auto'' margins to zero
 			and overflow as specified by their <a>box alignment properties</a>.
 	</ul>
 
@@ -5330,6 +5333,10 @@ Changes since the <a href="https://www.w3.org/TR/2020/CRD-css-grid-2-20201218/">
 			now that [=non-replaced elements=] can also have an aspect ratio
 			via 'aspect-ratio'. [[CSS-SIZING-4]]
 			(<a href="https://github.com/w3c/csswg-drafts/issues/6069">Issue 6069</a>)
+		<li id="change-2020-auto-margins">
+			Made [[#auto-margins]] normative and tightened up wording,
+			since some aspects of this behavior were not otherwise defined elsewhere.
+			(part of <a href="https://github.com/w3c/csswg-drafts/issues/5923">Issue 5923</a>)
 	</ul>
 
 <h3 id="changes-202008">