[selectors-4] Disallow pseudo-elements from :has unless on an allowlist. w3c#7463

Author: tabatkins

selectors-4/Overview.bs

@@ -1194,7 +1194,17 @@ The Relational Pseudo-class: '':has()''</h3>
 	would match at least one element
 	when [=anchor element|anchored against=] this element.
 
-	'':has()'' pseudo-class cannot be nested; '':has()'' is not valid within '':has()''.
+	The '':has()'' pseudo-class cannot be nested;
+	'':has()'' is not valid within '':has()''.
+	Also, unless explicitly defined as a
+	<dfn dfn export>:has-allowed pseudo-element</dfn>,
+	[=pseudo-elements=] are not valid selectors within '':has()''.
+	(This specification does not define any [=:has-allowed pseudo-elements=],
+	but other specifications may do so.)
+
+	Note: Pseudo-elements are generally excluded from '':has()'' because
+	many of them exist conditionally, based on the styling of their ancestors,
+	so allowing these to be queried by '':has()'' would introduce cycles.
 
 	<div class='example'>
 		For example, the following selector matches only