Allow UAs to conditionally block on stylesheet loading

In particular, if the media attribute doesn't match the environment there's no need to block on it.

Fixes #2886.

source

@@ -15082,15 +15082,30 @@ interface <dfn>HTMLStyleElement</dfn> : <span>HTMLElement</span> {
   to script once the <span>event loop</span> reaches its <span>update the rendering</span> step.</p>
 
   <p>A style sheet in the context of the <code>Document</code> of an <span>HTML parser</span> or
-  <span>XML parser</span> is said to be <dfn>a style sheet that is blocking scripts</dfn> if the
-  element was created by that <code>Document</code>'s parser, and the element is either a
-  <code>style</code> element or a <code>link</code> element that was an <span
-  data-x="rel-stylesheet">external resource link that contributes to the styling processing
-  model</span> when the element was created by the parser, and the element's style sheet was enabled
-  when the element was created by the parser, and the element's <span>style sheet ready</span> flag
-  is not yet set, and, the last time the <span>event loop</span> reached <a href="#step1">step 1</a>,
-  the element's <span>root</span> was that <code>Document</code>, and the user agent hasn't given up
-  on that particular style sheet yet. A user agent may give up on a style sheet at any time.</p>
+  <span>XML parser</span> is said to be <dfn>a style sheet that is blocking scripts</dfn> if all of
+  the following conditions occur:</p>
+
+  <ul>
+   <li><p>The element was created by that <code>Document</code>'s parser.</p></li>
+
+   <li><p>The element is either a <code>style</code> element or a <code>link</code> element that was
+   an <span data-x="rel-stylesheet">external resource link that contributes to the styling
+   processing model</span> when the element was created by the parser.</p></li>
+
+   <li><p>If the element is a <code>link</code> element, it's <code
+   data-x="attr-link-media">media</code> attribute's value <span>matches the
+   environment</span>.</p></li>
+
+   <li><p>The element's style sheet was enabled when the element was created by the parser.</p></li>
+
+   <li><p>The element's <span>style sheet ready</span> flag is not yet set.</p></li>
+
+   <li><p>The last time the <span>event loop</span> reached <a href="#step1">step 1</a>, the
+   element's <span>root</span> was that <code>Document</code>.</p></li>
+
+   <li><p>The user agent hasn't given up on that particular style sheet yet. A user agent may give
+   up on a style sheet at any time.</p></li>
+  </ul>
 
   <p class="note">Giving up on a style sheet before the style sheet loads, if the style sheet
   eventually does still load, means that the script might end up operating with incorrect