[cssom] Let elements in non-rendered iframes not expose styles.

Additional resolution in w3c#1548.

cssom-1/Overview.bs

@@ -35,6 +35,9 @@ urlPrefix: https://html.spec.whatwg.org/multipage/
             text: tree order
             text: document base url
             text: content-type metadata; url: #content-type
+    urlPrefix: rendering.html
+        type: dfn
+            text: being rendered
     urlPrefix: browsers.html
         type: interface; text: WindowProxy
         type: dfn
@@ -2780,7 +2783,10 @@ steps:
   match above.
 
  <li>Let <var>decls</var> be an empty list of <a>CSS declarations</a>.
- <li>If <var>elt</var> is <a>connected</a> and part of the <a>flat tree</a>,
+ <li>If <var>elt</var> is <a>connected</a>, part of the <a>flat tree</a>,
+  and its <a>shadow-including root</a>'s <a>browsing context</a> either doesn't have a
+  <a>browsing context container</a>, or that <a>browsing context container</a>
+  is <a>being rendered</a>,
   let <var>decls</var> be a list of all longhand properties that are
   <a lt="supported CSS property">supported CSS properties</a>, in lexicographical order,
   with the value being the <a>resolved value</a> computed for <var>obj</var>