[css-pseudo-4] Add RR diagram #5830

Author: fantasai

css-pseudo-4/Overview.bs

@@ -346,6 +346,27 @@ First-Letter Text: ''::first-letter'' pseudo-element and its ''::prefix'' and ''
     <!-- this is not actually BNF, but it highlights better that way -->
 
     ISSUE: Bikeshed is <a href="https://github.com/tabatkins/bikeshed/issues/2202">eating punctuation</a> in this note, which is not helpful.
+
+    <pre class=railroad>
+      Optional:
+        Sequence:
+          N: P
+          ZeroOrMore:
+            Choice:
+              N: P
+              N: Zs
+      Choice:
+        N: L
+        N: N
+        N: S
+      Optional:
+        Sequence:
+          N: P − (Ps ∪ Pd)
+          ZeroOrMore:
+            Choice:
+              N: P − (Ps ∪ Pd)
+              N: Zs
+    </pre>
   </div>
 
   <div class="figure">