[css-anchor-1] Allow anchoring to abspos anchors (w3c#8779)

Author: xiaochengh

css-anchor-position-1/Overview.bs

@@ -611,37 +611,20 @@ might not be capable of anchoring the positioned element.
 <div algorithm="acceptable anchor element">
 	An element |el| is a <dfn export>acceptable anchor element</dfn>
 	for an [=absolutely positioned=] element |query el|
-	if any of the following are true:
+	if all of the following are true:
 
-	* |query el| is in a higher [=root layer=] than |el|.
+	* Either |el| is a descendant of |query el|'s [=containing block=],
+		or |query el|'s [=containing block=] is the [=initial containing block=].
 
-	* |query el| and |el| are in the same [=root layer=],
-		and all of the following are true:
+	* If |el| has the same [=containing block=] as |query el|,
+		then either |el| is not [=absolutely positioned=],
+		or |el| precedes |query el| in the tree order.
 
-		* Either |el| is a descendant of |query el|'s [=containing block=],
-			or |query el|'s [=containing block=] is the [=initial containing block=].
-
-		* If |el| has the same [=containing block=] as |query el|,
-			|el| is not [=absolutely positioned=].
-
-		* If |el| has a different [=containing block=] from |query el|,
-			the last [=containing block=] in |el|'s [=containing block chain=]
-			before reaching |query el|'s [=containing block=]
-			is not [=absolutely positioned=].
-
-	For the purposes of this algorithm,
-	an element is in a particular <dfn noexport>root layer</dfn>
-	corresponding to the closest [=inclusive ancestor=]
-	that is in the [=top layer=],
-	or the document if there isn't one.
-	[=Root layers=] are "higher" if their corresponding element
-	is later in the [=top layer=] list;
-	the layer corresponding to the document is lower than all other layers.
-
-	Note: This wording around "root layer"
-	needs to live in a stacking-context spec,
-	after pulling the [=top layer=] stuff
-	out of [[fullscreen]].
+	* If |el| has a different [=containing block=] from |query el|,
+		then the last [=containing block=] in |el|'s [=containing block chain=]
+		before reaching |query el|'s [=containing block=]
+		is either not [=absolutely positioned=]
+		or precedes |query el| in the tree order.
 </div>
 
 An element can also have an <dfn export>implicit anchor element</dfn>,