[cssom] Restore some links that were accidentally lost in #6539

Author: emilio

cssom-1/Overview.bs

@@ -68,6 +68,9 @@ urlPrefix: https://html.spec.whatwg.org/multipage/
     urlPrefix: xhtml.html
         type: dfn
             text: xml parser
+    urlPrefix: semantics.html
+        type: dfn
+            text: a style sheet that is blocking scripts
 urlPrefix: https://dom.spec.whatwg.org/#concept-
     type: dfn
         text: dispatch; url: event-dispatch