[selectors-5] Add custom state pseudo-classes (#8213)

We resolved to add the custom state pseudo-classes to selectors 5 here:
#4805

Co-authored-by: Keith Cirkel <keithamus@users.noreply.github.com>

selectors-4/deferred-for-level-5

@@ -127,3 +127,14 @@ The local link pseudo-class '':local-link''</h3>
 		What about if the document url has less than N segments,
 		and the link is same-page?
 		Should "null segments" count as matching, or not?
+
+<h3 id="custom-state">Exposing custom state: the '':state()'' pseudo-class</h3>
+
+  The <dfn selector>:state()</dfn> pseudo-class takes a case-sensitive argument
+  and matches custom elements whose exposed custom states include the argument.
+
+  The grammar of the '':state()'' pseudo-class is:
+
+  <pre class=prod>:state( <ident> )</pre>
+
+  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>.