[css-conditional-5] Container queries use the flat tree w3c#5984 (w3c#10701)

Per resolution in:

w3c#5984 (comment)

Co-authored-by: Rune Lillesveen <futhark@chromium.org>

Author: lilles

css-conditional-5/Overview.bs

@@ -353,7 +353,7 @@ Container Queries</h2>
 	for [=container size queries=] by specifying
 	the additional query types using the 'container-type' property
 	(or the 'container' [=shorthand=]).
-	Style rules applying to a [=query container=]’s <a>shadow-including descendants</a>
+	Style rules applying to a [=query container=]’s [=flat tree=] descendants
 	can be conditioned by querying against it,
 	using the ''@container'' [=conditional group rule=].
 
@@ -383,30 +383,25 @@ Container Queries</h2>
 		will each respond to their own container context.
 	</div>
 
-	For selectors with pseudo elements, query containers can be established by
-	the <a>shadow-including inclusive ancestors</a> of the <a>ultimate
-	originating element</a>.
+	For the ''::part()'' and ''::slotted()'' <a>pseudo-element</a> selectors,
+	which represent real elements in the DOM tree, query containers can be
+	established by [=flat tree=] ancestors of those elements.
+	For other <a>pseudo-element</a>s, query containers can be established by
+	inclusive [=flat tree=] ancestors of their <a>originating element</a>.
 
 	<div class=note>
 		It follows that:
 
-		* Pseudo elements themselves can not be query containers
 		* ''::before'', ''::after'', ''::marker'', and ''::backdrop'' query their
 			originating elements
 		* ''::first-letter'' and ''::first-line'' query their originating elements,
 			even if the <a>fictional tag sequence</a> may push the
 			<code>::first-line</code> past other elements for the purpose of
 			inheritance and rendering
-		* Multiple pseudo elements do not allow pseudo elements to be query
-			containers for other pseudo elements. E.g., the host, but not the
-			<code>::part()</code>, can be the query container for
-			<code>::before</code> in <code>host::part()::before</code>. Similarly,
-			<code>::before</code> can not be the query container for the
-			<code>::marker</code> in <code>div::before::marker</code>
 		* ''::slotted()'' selectors can query containers inside the shadow tree,
 			including the slot itself
-		* ''::part()'' selectors can query its originating host, but not internal
-			query containers inside the shadow tree
+		* ''::slotted()::before'' selectors can query the slotted shadow host child
+		* ''::part()'' selectors can query containers inside the shadow tree
 		* ''::placeholder'' and ''::file-selector-button'' can query the input
 			element, but do not expose any internal containers if the input element is
 			implemented using a shadow tree