[css-align] Define how staticpos is calculated for abspos.

Author: tabatkins

css-align/Overview.bs

@@ -15,7 +15,7 @@ Previous Version: https://www.w3.org/TR/2013/WD-css3-align-20130514/
 Previous Version: https://www.w3.org/TR/2012/WD-css3-align-20120612/
 Abstract: This module contains the features of <a href="https://www.w3.org/TR/CSS/">CSS</a> relating to the alignment of boxes within their containers in the various CSS box layout models: block layout, table layout, flex layout, and grid layout. (The alignment of text and inline-level content is defined in [[CSS-TEXT-3]] and [[CSS-INLINE-3]].)
 Link Defaults: selectors-3 (dfn) first formatted line, css-flexbox-1 (dfn) flex line, css-multicol-1 (dfn) multi-column element, css-fonts-3 (dfn) first available font, css21 (property) max-width/max-height/min-width/min-height, css-writing-modes-3 (dfn) dominant baseline, css-position-3 (property) left, css-cascade-3 (value) initial
-Ignored Terms: table cell, stretch, scrollable overflow region
+Ignored Terms: table cell, stretch, scrollable overflow region, static position rectangle
 At Risk: ''last-baseline''
 </pre>
 
@@ -894,11 +894,17 @@ Inline/Main-Axis Alignment: the 'justify-self' property</h3>
 				Values other than ''justify-self/stretch'' cause <a href="https://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-width">non-replaced absolutely-positioned boxes</a>
 				to use <a>fit-content sizing</a> for calculating ''width/auto'' inline sizes.
 
-				Note that ''justify-self/stretch'' does cause replaced absolutely-positioned elements to fill their containing block
+				Note that ''justify-self/stretch'' <em>does</em> cause replaced absolutely-positioned elements to fill their containing block
 				just as non-replaced ones do.
 
 				If either offset property in this dimension is ''top/auto'',
 				'justify-self' has no effect.
+
+				For the purpose of determining the static position,
+				the <a>alignment container</a> is instead the <a>static position rectangle</a>
+				defined by the element's parent's layout mode,
+				and the element is treated as having zero offsets in every axis
+				(so this property <em>does</em> have an effect).
 	</table>
 
 ### Table Cells ### {#justify-cell}
@@ -1020,6 +1026,12 @@ Block/Cross-Axis Alignment: the 'align-self' property</h3>
 
 				If either offset property in this dimension is ''top/auto'',
 				'align-self' has no effect.
+
+				For the purpose of determining the static position,
+				the <a>alignment container</a> is instead the <a>static position rectangle</a>
+				defined by the element's parent's layout mode,
+				and the element is treated as having zero offsets in every axis
+				(so this property <em>does</em> have an effect).
 	</table>
 
 ### Table Cells ### {#align-cell}