[selectors-4] Mark issue about selector invalidation inside :is()

Author: fantasai

selectors-4/Overview.bs

@@ -1033,6 +1033,11 @@ Invalid Selectors and Error Handling</h3>
 		<li>an empty selector, i.e. one that contains no <a>compound selector</a>, is invalid.
 	</ul>
 
+	ISSUE: The CSSWG indends to modify these rules such that an invalid selector
+	in the selector list argument of a pseudo-class like `:is()`
+	is simply ignored rather than invalidating the whole selector list.
+	See <a href="https://github.com/w3c/csswg-drafts/issues/3264">Issue 3264</a>.
+
 	Note: Consistent with CSS’s forwards-compatible parsing principle,
 	UAs <em>must</em> treat as <a>invalid</a>
 	any pseudo-classes, pseudo-elements, combinators, or other syntactic constructs