[css-pseudo-4] Add reminder that chaining is only allowed if explicitly specified #7118

Author: fantasai

css-pseudo-4/Overview.bs

@@ -54,6 +54,11 @@ spec:css-backgrounds-3; type:property; text:box-shadow
   and on their syntax and interaction with other <a>selectors</a>,
   see [[!SELECTORS-4]].
 
+  Note: As a reminder,
+  pseudo-elements cannot be chained together unless explicitly allowed.
+  For example, ''::marker::before'' is not allowed;
+  but ''::before::marker'' is.
+
 <h2 id="typographic-pseudos">
 Typographic Pseudo-elements</h2>