[css-position-3] Rewrite abspos layout rules Part 1: define inset-modified containing block.

Author: fantasai

css-position-3/Overview.bs

@@ -19,6 +19,7 @@ Link Defaults: css-transforms-1 (property) transform
 </pre>
 
 <style type="text/css">
+	.switch > dt { font: inherit; }
 	.cb-example-table {
 		border: 1px solid;
 		border-collapse: collapse;
@@ -616,20 +617,57 @@ Sticky positioning</h3>
 <!-- End section: Sticky positioning -->
 
 <h3 id="abs-pos">
-Absolute positioning</h3>
+Absolute Positioning</h3>
 
 	For an [=absolutely positioned=] box,
 	the [=inset properties=] effectively reduce the [=containing block=]
-	into which it is sized and positioned.
+	into which it is sized and positioned
+	by the specified amounts.
+
+	If only one [=inset property=] in a given axis is ''top/auto'',
+	it is set to zero.
+	If both [=inset properties=] in a given axis are ''top/auto'', then,
+	depending on the box’s [=self-alignment property=] in the relevant axis:
+	<dl class=switch>
+		<dt>for ''align-self/self-start'' alignment or its equivalent
+		<dd>
+			Set its start-edge [=inset property=] to the static position,
+			and its end-edge [=inset property=] to zero.
+
+		<dt>for ''align-self/self-end'' alignment or its equivalent
+		<dd>
+			Set its end-edge [=inset property=] to the static position,
+			and its start-edge [=inset property=]  to zero.
+
+		<dt>for ''align-self/center'' alignment
+		<dd>
+			Let <var>start distance</var> be the distance from
+			the center of its [=static position rectangle=]
+			to the start edge of its [=containing block=],
+			and <var>end distance</var> be the distance from
+			the center of its [=static position rectangle=]
+			to the end edge of its [=containing block=].
+			If <var>start distance</var> is less than or equal to <var>end distance</var>,
+			then set the start-edge [=inset property=] to zero,
+			and set the end-edge [=inset property=] to
+			(<var>containing block size</var> - 2 &times; |<var>start distance</var>|);
+			otherwise,
+			set the end-edge [=inset property=] to zero
+			and the start-edge [=inset property=] to 
+			(<var>containing block size</var> - 2 &times; |<var>end distance</var>|).
+	</dl>
 
-	If this results in an effective [=containing block=] size in any axis
-	less than zero,
-	then the effective end-edge inset in the affected axis
+	If these adjustments result in
+	an effective [=containing block=] size in any axis less than zero,
+	then the weaker inset in the affected axis
 	is reduced to bring that size up to zero.
+	In the case that only one inset is ''auto'',
+	that is the weaker inset;
+	otherwise the weaker inset is the end-edge inset.
 
 	Sizing and positioning of the [=absolutely positioned box=]
 	into this <dfn>inset-modified containing block</dfn>
-	is as described in [[#size-and-position-details]].
+	is as described in [[#abspos-layout]].
 
 <h4 id="breaking">
 Fragmenting Absolutely-positioned Elements</h4>
@@ -690,8 +728,8 @@ Fixed positioning</h3>
 <!-- End section: Positioning schemes -->
 
 
-<h2 id="size-and-position-details">
-Sizing and positioning details</h2>
+<h2 id="abspos-layout">
+Absolute Positioning Layout Model</h2>
 
 <h3 id="abs-non-replaced-width">
 The Width of Absolutely-Positioned, Non-Replaced Elements</h3>