[selectors-4] Add :interest-source/:interest-target, per WG resolution. w3c#12154

Author: tabatkins

selectors-4/Overview.bs

@@ -2513,6 +2513,30 @@ The Focus Container Pseudo-class: '':focus-within''</h3>
 	(including non-element nodes, such as text nodes)
 	matches the conditions for matching '':focus''.
 
+<h3 id="interest-pseudos">
+The Interest Pseudo-classes: '':interest-source'' and '':interest-target''</h3>
+
+	[=Host languages=] can define that certain elements
+	are capable of "showing interest" in another element
+	(typically, an informative popup),
+	so that when a user interacts with the first element,
+	the second element can be shown or otherwise rendered differently.
+
+	Issue: TODO Add reference to the HTML feature when it's fully landed.
+
+	The <dfn>:interest-source</dfn> pseudo-class
+	applies to any element (or pseudo-element)
+	that is capable of "showing interest" in another element,
+	as defined by the [=host language=].
+	(It applies regardless of whether the element 
+	is <em>currently</em> showing interest or not.)
+
+	The <dfn>:interest-target</dfn> pseudo-class
+	applies to the "other element"
+	that an '':interest-source'' element is currently showing interest in.
+	(It only applies to an element 
+	that is <em>currently</em> having interest shown in it.)
+
 <h2 id="time-pseudos">
 Time-dimensional Pseudo-classes</h2>