Do not fetch stylesheets in documents without a browsing context

This fixes #1495 by changing the "appropriate times to obtain the
resource" to require that the <link rel="stylesheet"> element be in a
document that has a browsing context.

source

@@ -23212,34 +23212,39 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
 
   <p>The appropriate times to <span data-x="concept-link-obtain">obtain</span> the resource are:
 
+  <!-- has a browsing context: http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=4299 -->
   <ul>
 
    <li><p>When the <span>external resource link</span> is created on a <code>link</code> element
-   that is already <span>in a document</span>.</p></li>
+   that is already <span>in a document</span> that has a <span data-x="concept-document-bc">browsing
+   context</span>.</p></li>
    <!-- e.g. rel="" changed, href="" set... -->
 
    <li><p>When the <span>external resource link</span>'s <code>link</code> element is <span
-   data-x="node is inserted into a document">inserted into a document</span>.</p></li>
+   data-x="node is inserted into a document">inserted into a document</span> that has a <span
+   data-x="concept-document-bc">browsing context</span>.</p></li>
 
    <li><p>When the <code data-x="attr-link-href">href</code> attribute of the <code>link</code>
-   element of an <span>external resource link</span> that is already <span>in a document</span> is
-   changed.</p></li>
+   element of an <span>external resource link</span> that is already <span>in a document</span> that
+   has a <span data-x="concept-document-bc">browsing context</span> is changed.</p></li>
    <!-- http://software.hixie.ch/utilities/js/live-dom-viewer/saved/2588 -->
 
    <li><p>When the <code data-x="attr-link-crossorigin">crossorigin</code> attribute of the
    <code>link</code> element of an <span data-x="external resource link">external resource
-   link</span> that is already <span>in a document</span> is set, changed, or removed.</p></li>
+   link</span> that is already <span>in a document</span> that has a <span
+   data-x="concept-document-bc">browsing context</span> is set, changed, or removed.</p></li>
 
    <li><p>When the <code data-x="attr-link-type">type</code> attribute of the <code>link</code>
-   element of an <span>external resource link</span> that is already <span>in a document</span> is
-   set or changed to a value that does not or no longer matches the <span
-   data-x="Content-Type">Content-Type metadata</span> of the previous obtained external resource, if
-   any.</p></li>
+   element of an <span>external resource link</span> that is already <span>in a document</span> that
+   has a <span data-x="concept-document-bc">browsing context</span> is set or changed to a value
+   that does not or no longer matches the <span data-x="Content-Type">Content-Type metadata</span>
+   of the previous obtained external resource, if any.</p></li>
 
    <li><p>When the <code data-x="attr-link-type">type</code> attribute of the <code>link</code>
-   element of an <span>external resource link</span> that is already <span>in a document</span>, but
-   was previously not obtained due to the <code data-x="attr-link-type">type</code> attribute
-   specifying an unsupported type is set, removed, or changed.</p></li>
+   element of an <span>external resource link</span> that is already <span>in a document</span> that
+   has a <span data-x="concept-document-bc">browsing context</span>, but was previously not obtained
+   due to the <code data-x="attr-link-type">type</code> attribute specifying an unsupported type, is
+   set, removed, or changed.</p></li>
 
    <li><p>When the <span>external resource link</span> changes from being <span data-x="the link is
    an alternative stylesheet">an alternative stylesheet</span> to not being one, or vice