[selectors-4] Define that featureless elements can be matched by logical combinations.

Author: tabatkins

selectors-4/Overview.bs

@@ -636,13 +636,17 @@ Data Model</h3>
 	While individual elements may lack any of the above features,
 	some elements are <dfn export>featureless</dfn>.
 	A <a>featureless</a> element does not match any selector at all,
-	except those it is explicitly defined to match.
+	except those it is explicitly defined to match,
+	plus the [=logical combination pseudo-classes=].
 	If a given selector <em>is</em> allowed to match a <a>featureless</a> element,
 	it must do so while ignoring the default namespace. [[CSS3NAMESPACE]]
 
 	<div class='example'>
 		For example, the <a>shadow host</a> in a <a>shadow tree</a> is <a>featureless</a>,
-		and can't be matched by <em>any</em> <a>pseudo-class</a> except for '':host'' and '':host-context()''.)
+		and can't be matched by <em>any</em> <a>pseudo-class</a>
+		except for '':host'' and '':host-context()''
+		(or combinations including those,
+		such as '':is(:host, .foo)'').
 	</div>
 
 	Many of the selectors depend on the semantics of the <dfn export>document language</dfn>