[selectors-4] Define that :hover/etc match on the *flat-tree* ancestors, up to the first top-layer element. w3c#11185

Author: tabatkins

selectors-4/Overview.bs

@@ -2318,7 +2318,7 @@ User Action Pseudo-classes</h2>
 	for the selection of an element the user is acting on.
 	(In non-interactive user agents, these pseudo-classes are valid, but never match any element.)
 
-	These pseudo-classes are not mutually exclusive.
+	The [=user action pseudo-classes=] are not mutually exclusive.
 	An element can match several such pseudo-classes at the same time.
 
 	<div class="example">
@@ -2333,6 +2333,16 @@ User Action Pseudo-classes</h2>
 		</pre>
 	</div>
 
+	Some [=user action pseudo-classes=],
+	in addition to matching on the particular element with the property in question,
+	match on that element's ancestors as well:
+	'':hover'', '':active'', '':focus-within''.
+	Specifically, if these match on a given element,
+	they also match on the element's [=flat-tree=] ancestors,
+	up to and including the first [=top layer=] element
+	or the [=root=] element,
+	whichever is encountered first.
+
 	Note: The specifics of hit-testing,
 	necessary to know when several of the pseudo-classes defined in this section apply,
 	are not yet defined,