[css-anchor-position-1] Use last valid anchor, not first. w3c#8165

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -570,7 +570,7 @@ might not be capable of anchoring the positioned element.
 		Otherwise, return nothing.
 
 	3. Otherwise, |anchor spec| is a <<dashed-ident>>.
-		Return the first element |el| in tree order
+		Return the last element |el| in tree order
 		that satisfies the following conditions:
 
 		* |el| is an [=anchor element=] with an [=anchor name=] of |name|.