Fix #568: avoid setting the url for blob URLs only

This hopefully makes it a little clearer what scenarios have observable
side effects.

source

@@ -20398,7 +20398,8 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
   data-noexport="">reinitialise url</dfn> algorithm, which runs these steps:</p>
 
   <ol>
-   <li><p>If element's <span data-x="concept-hyperlink-url">url</span> is non-null and its
+   <li><p>If element's <span data-x="concept-hyperlink-url">url</span> is non-null, its <span
+   data-x="concept-url-scheme">scheme</span> is "<code data-x="">blob</code>", and its
    <span>non-relative flag</span> is set, terminate these steps.</p></li>
 
    <li><p><span data-x="concept-hyperlink-url-set">Set the url</span>.</p></li>