[css-pseudo-4] Clarify that applying properties to ::first-letter can't change what matches ::first-letter. w3c#4392

Author: fantasai

css-pseudo-4/Overview.bs

@@ -196,6 +196,10 @@ Styling the First Line Pseudo-element</h4>
     <li>'text-orientation'
   </ul>
 
+  However,
+  in no case may the application of such unlisted properties to ''::first-letter'' change
+  what [=first-letter text=] is represented by that ''::first-letter''.
+
 <h4 id="first-line-inheritance">
 Inheritance and the ''::first-line'' Pseudo-element</h4>