[css-anchor-position-1] Loose matching for target anchor name (#12941)

97eef7b intended to allow
cross-shadow anchoring, but didn't adjust the same-tree-root
requirement in "determine target anchor element".

Author: andruud

css-anchor-position-1/Overview.bs

@@ -421,7 +421,7 @@ in anchor positioning.
 
 		* |el| is an [=anchor element=] with an [=anchor name=] of |anchor spec|.
 
-		* |el|'s [=anchor name=] and |anchor spec| are both associated with the same [=tree=] [=tree/root=].
+		* |el|'s [=anchor name=] [=tree-scoped name/loosely matched|loosely matches=] |anchor spec|.
 
 			Note: The [=anchor name=] is a [=tree-scoped name=],
 			while |anchor spec| is a [=tree-scoped reference=].