Add normative text for the script-blocking style sheet set

Author: domfarolino

cssom-1/Overview.bs

@@ -71,6 +71,9 @@ urlPrefix: https://html.spec.whatwg.org/multipage/
     urlPrefix: semantics.html
         type: dfn
             text: a style sheet that is blocking scripts
+            text: contributes a script-blocking style sheet
+            for: Document
+                text: script-blocking style sheet set
 urlPrefix: https://dom.spec.whatwg.org/#concept-
     type: dfn
         text: dispatch; url: event-dispatch
@@ -1540,6 +1543,11 @@ must be run:
      </ol>
    </ol>
  </ol>
+
+If <var>node</var> <a>contributes a script-blocking style sheet</a>, then user agents must <a
+for=list>append</a> <var>node</var> to its <a>node document</a>'s <a for=Document>script-blocking
+style sheet set</a>.
+
 </div>
 
 ### Requirements on user agents Implementing the HTTP Link Header ### {#requirements-on-user-agents-implementing-the-http-link-header}