[selectors-4] Expand text on :user-error issue.

Author: fantasai

selectors/Overview.bs

@@ -2313,13 +2313,28 @@ The User-interaction Pseudo-class: '':user-error''</h4>
 		</pre>
 	</div>
 
-<!--
-	<p class="issue">
-		If it's necessary to distinguish between omitted and invalid values,
+	<div class="issue">
+		This pseudo-class, which was taken from the
+		<a href="https://developer.mozilla.org/en-US/docs/Web/CSS/%3A-moz-ui-invalid">'':-moz-ui-invalid''</a> selector,
+		is named '':user-error'' rather than '':user-invalid''
+		because it combines checks for validity as well as omission.
+		We thought if it became necessary to distinguish between omitted and invalid values,
 		'':user-invalid'' and '':user-omitted'' could be used.
--->
 
-	Issue: Look into :-moz-ui-valid as well.
+		Alternatively, it might make more sense to rename this to '':user-invalid''
+		and add a '':blank'' pseudo-class that meant the field was blank.
+		This would allow authors to combine '':user-invalid:blank'' to select omitted required values,
+		and to combine '':user-invalid:not(:blank)'' to select filled out invalid values,
+		if necessary.
+
+		Both options are reasonable;
+		however since I can't think of a name for the '':-moz-ui-valid''
+		that would reasonably parallel '':user-error'',
+		I'm leaning somewhat to the second option.
+		(Other opinions welcome.)
+	</div>
+
+	Issue: Look into standardizing <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/%3A-moz-ui-valid">:-moz-ui-valid</a> as well.
 
 	Issue: Evaluate proposed <a href="https://lists.w3.org/Archives/Public/www-style/2014Feb/0511.html">:dirty pseudo-class</a>