diff --git a/selectors-4/Overview.bs b/selectors-4/Overview.bs
index ac0f8b67ca3f..7b2f815dd935 100644
--- a/selectors-4/Overview.bs
+++ b/selectors-4/Overview.bs
@@ -3726,6 +3726,11 @@ Grammar</h2>
 			(This might be empty.)
 	</div>
 
+	Any items in a <<forgiving-selector-list>> that are invalid
+	(whether explicitly, by using unknown selectors or syntax,
+	or merely contextually, using known syntax but in an invalid context)
+	must be treated as having zero specificity.
+
 	Note: <<forgiving-selector-list>> is intentionally used only in '':is()'' and '':where()'',
 	not in any other selector that takes a selector argument.
 	<!--