[css-anchor-1] Check the proper trees, so references work as expected. #7916

Author: tabatkins

css-anchor-1/Overview.bs

@@ -321,18 +321,27 @@ might not be capable of anchoring the positioned element.
 		2. Otherwise, return nothing.
 	2. Otherwise, return the first element |el| in tree order
 		that satisfies the following conditions:
-		* |el| and |query el| have the same [=tree/root=].
+
 		* |el| is an [=anchor element=] with an [=anchor name=] of |name|.
+
+		* |el|'s [=anchor name=] and |name| are both associated with the same [=tree=] [=tree/root=].
+
+			Note: The [=anchor name=] is a [=tree-scoped name=],
+			while |name| is a [=tree-scoped reference=].
+
 		* 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=]
 
-	Note: There might not be any such element.
+		If no element satisfies these conditions,
+		return nothing.
 
 	Note: The general rule captured by these conditions
 	is that |el| must be fully laid out