[css-anchor-position] Containment doesn't scope anchor names, but content-skipping does. But anchoring prevents content-skipping from activating. #10040

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -148,11 +148,21 @@ Not all elements are capable of being [=anchor elements=]
 for a given positioned element,
 so a name can be reused in multiple places
 if the usages are scoped appropriately.
-If the element has either [=style containment=] or [=layout containment=],
-the 'anchor-name' property is [=scoped property|scoped=] [=scoped to a sub-tree|to a sub-tree=].
-If there are still multiple valid [=anchor elements=]
-with the given [=anchor name=],
-the last one is chosen.
+
+Note: If multiple elements share an [=anchor name=]
+and are all visible to a given positioned element,
+the [=target anchor element=] will be the last one in DOM order.
+'anchor-scope' can be used to limit what names are visible to a given element.
+
+[=Anchor names=] are <em>not</em> scoped by [=containment=] by default;
+even if an element has [=style containment|style=] or [=layout containment=]
+(or any similar sort of containment),
+the [=anchor names=] of its descendants are visible to elements elsewhere in the page.
+
+Note: While an element is in the [=skipped contents=] of another element
+(due to ''content-visibility: hidden'', for instance),
+it's not an [=acceptable anchor element=],
+effectively acting as if it had no names.
 
 ### Implicit Anchor Elements ### {#implicit}
 
@@ -371,6 +381,8 @@ or nothing (a missing specifier).
 		Issue: define the term that actually means this,
 		matching ::before/after/backdrop/etc
 		(but not ::marker/placeholder/etc whose box is not detectable).
+
+	* |el| is not in the [=skipped contents=] of another element.
 </div>
 
 <!-- Big Text: default
@@ -433,6 +445,21 @@ in ''anchor()'' and ''anchor-size()''.
 	</pre>
 </div>
 
+<h3 id=anchor-relevance>
+Anchor Relevance</h3>
+
+As long as an [=anchor element=]
+is a [=target anchor element=]
+for any positioned element
+that is [=relevant to the user=],
+the [=anchor element=] is also [=relevant to the user=].
+
+Note: This means that, for example,
+an anchor in a ''content-visibility: auto'' subtree
+will prevent its subtree from [=skipping its contents=]
+as long as the positioned element relying on it
+is [=relevant to the user=].
+
 
 Anchor-Based Positioning {#positioning}
 ========================