[css-anchor-position] Define single-auto inset behavior. w3c#12512

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1318,7 +1318,7 @@ Anchor-Based Alignment {#alignment}
 <h3 id=position-area-alignment>
 Area-specific Default Alignment</h3>
 
-When using 'position-area',
+When 'position-area' is not ''position-area/none'',
 the behavior of the ''align-self/normal'' [=self-alignment=] value
 changes depending on the <<position-area>> value,
 to align the box towards the anchor:
@@ -1332,6 +1332,14 @@ to align the box towards the anchor:
 	if it's specifying the “start” track of its axis,
 	the default alignment in that axis is ''align-self/end''; etc.
 
+However, if only one [=inset property=] in the relevant axis is ''inset/auto'',
+the default alignment is instead
+towards the edge with the non-''inset/auto'' inset;
+and this is an ''align-self/unsafe'' alignment.
+
+Note: This single-''inset/auto'' behavior preserves the way
+a single specified inset controls the position of an [=absolutely positioned=] box.
+
 <div class=example>
 	For example, assuming an English-equivalent writing mode (horizontal-tb, ltr),
 	then the value ''span-x-start top'' resolves to