[css-scoping-1][css-anchor-position-1] Clarify how anchor-scope works across tree scopes. #10526

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -277,6 +277,11 @@ Values have the following meanings:
 		to be in scope only for this element's descendants;
 		and limits descendants to only match these [=anchor names=]
 		to [=anchor elements=] within this subtree.
+		The <<dashed-ident>> is a [=tree-scoped name=].
+
+		Note: Per [[css-scoping-1]],
+		when two [=tree-scoped names=] are compared,
+		they only match if they're from the same tree.
 </dl>
 
 This property has no effect on [=implicit anchor elements=].

css-scoping-1/Overview.bs

@@ -724,6 +724,13 @@ Name-Defining Constructs and Inheritance</h3>
 	(In other words, [=tree-scoped names=] "inherit" into descendant shadow trees,
 	so long as they don't define the same name themselves.)
 
+	If two [=tree-scoped names=] are directly compared
+	(for example, comparing an [=anchor name=] with an 'anchor-scope' ident),
+	they are considered to match only if their identifiers match,
+	<em>and</em> their [=root=]s match exactly.
+	(If one has a [=root=] that's an ancestor of the other, for example,
+	they <em>do not</em> match.)
+
 	<div class=issue>
 		TODO: Fix all the at-rules that define global names,
 		and the properties that reference them,