[css-anchor-1] Clarify that implicit anchor elements still need to have the right layout relationship to the querying element.

Author: tabatkins

css-anchor-1/Overview.bs

@@ -316,9 +316,13 @@ might not be capable of anchoring the positioned element.
 	and an optional anchor name |name|:
 
 	1. If |name| is not provided:
+
 		1. If |query el| has an [=implicit anchor element=],
+			and that element is an [=acceptable anchor element=] for |query el|,
 			return that element.
+
 		2. Otherwise, return nothing.
+
 	2. Otherwise, return the first element |el| in tree order
 		that satisfies the following conditions:
 
@@ -329,16 +333,7 @@ might not be capable of anchoring the positioned element.
 			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=]
+		* |el| is an [=acceptable anchor element=] for |query el|.
 
 		If no element satisfies these conditions,
 		return nothing.
@@ -366,6 +361,23 @@ might not be capable of anchoring the positioned element.
 	but the details of that will depend on the API assigning them.)
 </div>
 
+<div algorithm>
+	An element |el| is a <dfn export>acceptable anchor element</dfn>
+	for an [=absolutely positioned=] element |query el|
+	if all of the following properties apply:
+
+	* 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=].
+</div>
+
 An element can also have an <dfn export>implicit anchor element</dfn>,
 used when an [=anchor function=] doesn't specify an explicit [=anchor name=].