[selectors-4] Fix :interest-source to only match the active source, not a potential source. Also rewrite to provide some dfn hooks for HTML to refer to.

Author: tabatkins

selectors-4/Overview.bs

@@ -2516,26 +2516,48 @@ The Focus Container Pseudo-class: '':focus-within''</h3>
 <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.
+	A common UI feature is the ability for the user to "show interest" in certain elements
+	(an <dfn export>interest source</dfn>),
+	and the UI to respond to that interest
+	by showing additional information in another element
+	(the <dfn export>interest target</dfn>),
+	typically a popup.
+	For example, hovering a username on a page
+	might bring up an information card for that user;
+	or long-pressing a link
+	might bring up a preview of the link's destination.
+	Whenever this happens, the first element
+
+	Note: Exactly what elements are capable of being [=interest sources=],
+	how they're linked to [=interest targets=],
+	and how "interest" is shown
+	are all [=host language=]-defined.
+	In HTML, for example,
+	this is done with the <{html-global/interestfor}> attribute
+	to indicate an element capable of "showing interest" 
+	(being an [=interest source=],
+	and that attribute points to another element
+	intended to show the additional information
+	(the associated [=interest target=])
+	(usually a <{html-global/popover}> element).
 
 	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.)
+	applies to an [=interest source=] element that the user is currently "showing interest" in,
+	and the <dfn>:interest-target</dfn> pseudo-class
+	applies to the associated [=interest target=]
+	of an element matching '':interest-source''.
+
+	Note: '':interest-source'' only matches an [=interest source=]
+	the user is <em>currently</em> interested in,
+	not any element that can <em>potentially</em> have interest shown in it.
+	In HTML, for example,
+	just having the <{html-global/interestfor}> attribute
+	does not make an element match this pseudo-class;
+	the user has to actually indicate interest in it too.
+	Similarly, '':interest-target'' only matches an [=interest target=]
+	that is linked to an element the user is <em>currently</em> interested in,
+	not any element pointed to by an <{html-global/interestefor}> attribute.
+
 
 <h2 id="time-pseudos">
 Time-dimensional Pseudo-classes</h2>