[css-position-3] Be consistent with css-align-3 that auto insets are …

…stronger than self-alignment #11902

css-position-3/Overview.bs

@@ -747,7 +747,8 @@ Resolvings Insets for the “Inset-Modified Containing Block”</h4>
 	is reduced (possibly becoming negative)
 	to bring that size up to zero.
 	In the case that only one inset is ''auto'',
-	that is the <dfn>weaker inset</dfn>;
+	that is the <dfn>weaker inset</dfn>
+	(whose opposite inset is the <dfn>stronger inset</dfn>);
 	otherwise the [=weaker inset=] is the inset of the [=end=] edge
 	(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
 
@@ -901,15 +902,8 @@ Absolute Positioning Layout Model</h2>
 			see [[#abspos-margins]].
 
 		<li>
-			Lastly, its [=margin box=] is aligned within the [=inset-modified containing block=]
-			as defined by its [=self-alignment properties=].
-
-			For this purpose, if neither ''<overflow-position>/safe'' nor ''<overflow-position>/unsafe'' is specified,
-			the box aligns at an “in-between” level of safety:
-			it is ''unsafe''-aligned within its [=inset-modified containing block=],
-			but shifted, if necessary, to not overflow
-			the union of its [=original containing block=] and its [=inset-modified containing block=]--
-			unless it is larger than that rectangle, in which case it is start-aligned to it.
+			Lastly, its [=margin box=] is aligned within the [=inset-modified containing block=],
+			see [[#abspos-alignment]].
 	</ol>
 
 <h3 id="abspos-auto-size">
@@ -975,6 +969,27 @@ Auto Margins of Absolutely-Positioned Boxes</h3>
 	the space distributed to ''margin/auto'' [=margins=]
 	can be negative in [=absolute positioning=].
 
+<h2 id=abspos-alignment>
+Self-Alignment of Absolutely Positioned Boxes</h2>
+
+	If the [=computed value=] of either [=inset property=] in an axis is ''inset/auto'',
+	then its [=margin box=] is aligned to the edge of the [=inset-modified containing block=]
+	corresponding to its [=stronger inset=]
+	(even if this would overflow its [=containing block=]).
+
+	Otherwise, if either [=margin=] is ''margin/auto'',
+	its position is resolved according to [[#abspos-margins]].
+
+	Otherwise, the box is aligned as specified by
+	its [=self-alignment property=] in the relevant axis
+	(as defined by the [=writing mode=] of the [=containing block=]),
+	using its [=margin box=] as the [=alignment subject=]
+	and the [=inset-modified containing block=] as the [=alignment container=].
+	However, if an explicit [=overflow alignment=] is not specified
+	and its [=margin box=] overflows the [=inset-modified containing block=],
+	its alignment is adjusted to minimize overflow
+	as specified in [[css-align-3#auto-safety-position]].
+
 <h2 id="abspos-old">
 Old Absolute Positioning Layout Model</h2>