Fix #1548. (#2563)

* [cssom] Make elements outside of the flat tree or detached not expose styles.

Fixes #1548

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

Additional resolution in #1548.

* fixup! [cssom] Make elements outside of the flat tree or detached not expose styles.

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

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
@@ -2779,19 +2782,27 @@ steps:
   versions. That is, both <code>:before</code> and <code>::before</code> should
   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>, part of the <a>flat tree</a>,
+  and its <a>shadow-including root</a> has a <a>browsing context</a> which either
+  doesn't have a <a>browsing context container</a>, or whose
+  <a>browsing context container</a> is <a>being rendered</a>,
+  set <var>decls</var> to 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>
+  using the style rules associated with <var>doc</var>.
+
+  Issue: There are UAs that handle shorthands, and all UAs handle shorthands
+  that used to be longhands like 'overflow', see
+  <a href="https://github.com/w3c/csswg-drafts/issues/2529">#2529</a>.
+
  <li>
   Return a live <a>CSS declaration block</a> with the following properties:
   <dl>
    <dt><a for="CSSStyleDeclaration">readonly flag</a>
    <dd>Set.
    <dt><a for="CSSStyleDeclaration">declarations</a>
-   <dd>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> using the style rules associated with <var>doc</var>.
-
-   Issue: There are UAs that handle shorthands, and all UAs handle shorthands
-   that used to be longhands like 'overflow', see
-   <a href="https://github.com/w3c/csswg-drafts/issues/2529">#2529</a>.
-
+   <dd><var>decls</var>.
    <dt><a for="CSSStyleDeclaration">parent CSS rule</a>
    <dd>Null.
    <dt><a for="CSSStyleDeclaration">owner node</a>