[css-scoping][selectors] Make featureless elements match *nothing*. Make Scoping clearer about what is allowed to match the host element.

Author: tabatkins

css-scoping/Overview.bs

@@ -277,7 +277,7 @@ Host Elements in a Shadow Tree</h4>
 
 	When considered within its own <a>shadow trees</a>,
 	the <a>host element</a> is <a>featureless</a>.
-	(That is, <a>feature selectors</a> like class, tagname, or even ''*'' never match it.)
+	Only the '':host'', '':host()'', and '':host-context()'' pseudo-classes are allowed to match it.
 
 	<details class='why'>
 		<summary>Why is the shadow host so weird?</summary>

selectors/Overview.bs

@@ -668,45 +668,18 @@ Data Model</h3>
 		<li>Attributes, which are pairs of strings consisting of an attribute name and an attribute value.
 	</ul>
 
-	These aspects are referred to as <dfn export lt="feature">features</dfn>.
-	The <a>simple selectors</a> which are keyed off of them
-	(<a>universal selector</a>, <a>type selectors</a>, <a>ID selectors</a>, <a>class selectors</a>, and <a>attribute selectors</a>)
-	are called <dfn export lt="feature selector">feature selectors</dfn>.
-
-	While elements may lack any of the individual <a>features</a>,
+	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 <a>feature selector</a>,
-	or any selector that resolves based on <a>features</a>
-	(such as '':not(div)''),
-	with the exception of namespace selectors--
-	a <a>featureless</a> element is treated as having <em>all</em> namespaces.
-	Thus, only <a>pseudo-classes</a> or <a>pseudo-elements</a> can be used to select them.
-	Individual <a>featureless</a> elements may define additional restrictions on what kinds of selectors can match them.
+	A <a>featureless</a> element does not match any selector at all,
+	except those it is explicitly defined to match.
+	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>host element</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()''.)
 	</div>
 
-	<details class='why'>
-		<summary>Why do featureless elements match all namespaces?</summary>
-
-		In a previous version of Selectors,
-		all <a>compound selectors</a> contained a <a>type selector</a>;
-		if you didn't supply one explicitly,
-		a <a>universal selector</a> was implied.
-		Default namespaces [[CSS3NAMESPACE]] affected the implied universal selector,
-		so that selectors like '':hover'' actually meant ''defaultNS|*:hover''.
-
-		The universal selector is no longer implied,
-		but default namespaces still exist,
-		and have to apply to all compound selectors.
-		The behavior of <a>featureless</a> elements,
-		like the <a>host element</a> in a shadow tree,
-		thus needs to accommodate that and work appropriately.
-		In this case, "appropriately" means "ignoring the default namespace".
-	</details>
-
 	When matching a selector against a document which is in <a>quirks mode</a>,
 	class and ID selectors must be matched <a lt="ASCII case-insensitive">ASCII case-insensitively</a>
 	against the classes and ID of the elements in the document.