[css-position-3] Rewrite Abspos layout model section.

Author: fantasai

css-position-3/Overview.bs

@@ -669,6 +669,13 @@ Absolute Positioning</h3>
 	into this <dfn>inset-modified containing block</dfn>
 	is as described in [[#abspos-layout]].
 
+	If its [=self-alignment property=] in an axis is ''align-self/normal'',
+	then the [=resolved value=] of its [=weaker inset=] in that axis
+	is the value necessary to match that edge of its [=inset-modified containing block=]
+	to the corresponding edge of its [=margin box=]
+	after [[#abspos-layout|layout]].
+	(Otherwise the [=resolved value=] is the [=used value=] described above.)
+
 <h4 id="abspos-breaking" oldids="breaking">
 Fragmenting Absolutely-positioned Elements</h4>
 
@@ -725,10 +732,75 @@ Fixed positioning</h3>
 <!-- End section: Fixed positioning -->
 <!-- End section: Positioning schemes -->
 
-
 <h2 id="abspos-layout">
 Absolute Positioning Layout Model</h2>
 
+	[=Absolute positioning=] not only takes a box out of flow,
+	but also lays it out in its [=containing block=]
+	(after the final size of the [=containing block=] has been determined)
+	according to the <dfn lt="absolute positioning layout | absolute postioning layout model">absolute positioning layout model</dfn>:
+
+	<ol>
+		<li>
+			First, its [=inset-modified containing block=] is calculated,
+			defining its [=available space=].
+			(See [[#abspos-insets]].)
+			Because an [=absolutely positioned box=] can have no effect on the size of its [=containing block=],
+			its [=available space=] is always [=definite=].
+		<li>
+			Next, its width and height are resolved against this [=definite=] [=available space=],
+			as its [=preferred size=] capped by its [=maximum size=] (if any), floored by its [=minimum size=].
+			See [[#abspos-auto-size]].
+
+		<li>
+			Then, the value of any ''margin/auto'' margins are calculated,
+			see [[#abspos-margins]].
+
+		<li>
+			Lastly, its [=margin box=] is aligned within the [=inset-modified containing block=]
+			as defined by its [=self-alignment properties=].
+	</ol>
+
+<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):
+
+	* If its [=self-alignment property=] in the relevant axis is ''align-self/stretch'',
+		or if it is ''align-self/normal'' and the box is [=non-replaced=],
+		its [=automatic size=] is its [=stretch-fit size=].
+
+	* If its [=self-alignment property=] in the relevant axis is not ''align-self/stretch'',
+		or if it is ''align-self/normal'' and the box is [=replaced=],
+		its [=automatic size=] is its [=fit-content size=].
+
+	The [=automatic minimum size=] of an absolutely-positioned box is always zero.
+
+<h3 id="abspos-margins">
+Auto Margins of Absolutely-Positioned Boxes</h3>
+
+	If either [=inset property=] in the relevant axis is ''top/auto'',
+	then any ''margin/auto'' [=margins=] resolve to zero.
+
+	Otherwise, the <var>remaining space</var> is calculated
+	as the size of its [=inset-modified containing block=] in the relevant axis
+	minus the box's used size in the relevant axis,
+	and this <var>remaining space</var> is divided among
+	any ''margin/auto'' margins in the relevant axis.
+	However, if the <var>remaining space</var> is negative
+	and both margins are ''margin/auto'',
+	the start margin resolves to zero
+	and the end margin receives the <var>remaining space</var>.
+
+<h2 id="abspos-old">
+Old Absolute Positioning Layout Model</h2>
+
+	ISSUE: This section is being replaced with the new [[#abspos-layout]] section.
+	It is preserved here for comparison:
+	both models should yield the same result
+	in [=horizontal writing modes=] when the box’s [=self-alignment=] is ''align-self/normal''.
+
 <h3 id="abs-non-replaced-width">
 The Width of Absolutely-Positioned, Non-Replaced Elements</h3>