[selectors-5][editorial] Light rewrite of :state() definition.

Author: tabatkins

selectors-5/Overview.bs

@@ -12,8 +12,10 @@ Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
 Abstract: <a>Selectors</a> are patterns that match against elements in a tree, and as such form one of several technologies that can be used to select nodes in a document. Selectors have been optimized for use with HTML and XML, and are designed to be usable in performance-critical code. They are a core component of <abbr title="Cascading Style Sheets">CSS</abbr> (Cascading Style Sheets), which uses Selectors to bind style properties to elements in the document.
 Abstract: Selectors Level 5 describes the selectors that already exist in [[!selectors-4]], and further introduces new selectors for CSS and other languages that may need them.
 </pre>
+
 <pre class="link-defaults">
 spec:selectors-4; type:dfn; text:selector
+spec:html; type:dfn; text:states set
 </pre>
 
 <h2 id="intro">
@@ -133,16 +135,24 @@ The local link pseudo-class '':local-link''</h3>
 <h2 id="custom-state">
 Exposing custom state: the '':state()'' pseudo-class</h2>
 
-	The <dfn selector>:state()</dfn> pseudo-class takes a case-sensitive argument
-	and matches custom elements whose exposed custom states include the argument.
+	The <dfn selector id="state-pseudo">:state( <<ident>> )</dfn> pseudo-class
+	matches [=custom elements=]
+	whose [=states set=] contains a string
+	that [=string/is=] the selector's argument's value.
 
-	The grammar of the '':state()'' pseudo-class is:
+	<!-- Using <<ident>>, rather than <<custom-ident>>,
+	     to avoid excluding the CSS-wide keywords. -->
 
-	<pre class=prod>:state( <ident> )</pre>
+	Note: The "[=string/is=]" matching behavior compares strings by codepoint;
+	notably, it's case-sensitive.
+	So if "foo" is in the [=states set=], '':state(FOO)'' will <em>not</em> match.
 
 	The exact matching behavior of '':state()'' pseudo-class
 	is defined by the host language.
-	See <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-state-pseudo-class">HTML's definition</a>.
+	For clarity, the concepts explaining this pseudo-class
+	link to the HTML definition;
+	see <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-state-pseudo-class">HTML's definition</a> for more detail.
+	Other host languages must define how this pseudo-class matches.
 
 <h2 id="combinators">
 Combinators</h2>