[selectors] Add :playing and :paused, per WG resolution.

Author: tabatkins

selectors/Overview.bs

@@ -2171,6 +2171,31 @@ The Future-element Pseudo-class: '':future''</h3>
 	sibling of a '':current'' element.
 
 
+<h2 id='resource-pseudos'>
+Resource State Pseudos</h2>
+
+	The pseudo-classes in this section apply to elements that represent loaded resources,
+	particularly images/videos,
+	and allow authors to select them based on some quality of their "state".
+
+<h3 id="video-state">
+Video/Audio Play State: the '':playing'' and '':paused'' pseudo-classes</h3>
+
+	The <dfn>:playing</dfn> pseudo-class represents an element
+	representing an audio, video, or similar resource
+	that is capable of being "played" or "paused",
+	when that element is "playing".
+	(This includes both when the element is explicitly playing,
+	and when it's temporarily stopped for some reason not connected to user intent,
+	but will automatically resume when that reason is resolved,
+	such as a "buffering" state.)
+
+	The <dfn>:paused</dfn> pseudo-class represents the same elements,
+	but instead match when the element is <em>not</em> "playing".
+	(This includes both an explicit "paused" state,
+	and other non-playing states like "loaded, hasn't been activated yet", etc.)
+
+
 <h2 id='input-pseudos'>
 The Input Pseudo-classes</h2>