Ignore a URL with an unsupported scheme (#447)

* Address #446

- Discard a URL with an unsupported scheme in constructing a PresentationRequest

* minor revision

index.html

@@ -422,6 +422,11 @@ <h2>
           "https://www.w3.org/TR/html51/browsers.html#parse-the-sandboxing-directive">
           parse a sandboxing directive</a></dfn>
         </li>
+        <li>
+          <dfn><a href=
+          "https://www.w3.org/TR/html51/infrastructure.html#reparsed">parse a
+          URL</a></dfn>
+        </li>
         <li>
           <dfn><a href=
           "https://www.w3.org/TR/html51/webappapis.html#queuing">queue a
@@ -1180,15 +1185,22 @@ <h4>
             </li>
             <li>For each URL <var>U</var> in <var>urls</var>:
               <ol>
-                <li>Resolve <var>U</var> relative to the API base URL specified
-                by the <a>current settings object</a>, and add the resulting
-                absolute URL (if any) to <var>presentationUrls</var>.
+                <li>let <var>A</var> be an absolute URL that is the result of
+                parsing <var>U</var> relative to the API base URL specified by
+                the <a>current settings object</a>.
                 </li>
-                <li>If the resolve a URL algorithm failed, then <a>throw</a> a
-                <a>SyntaxError</a> exception and abort all remaining steps.
+                <li>If the <a>parse a URL</a> algorithm failed, then
+                <a>throw</a> a <a>SyntaxError</a> exception and abort all
+                remaining steps.
+                </li>
+                <li>If <var>A</var>'s scheme is supported by the <a>controlling
+                user agent</a>, add <var>A</var> to <var>presentationUrls</var>.
                 </li>
               </ol>
             </li>
+            <li>If <var>presentationUrls</var> is an empty list, then throw a
+            <a>NotSupportedError</a> and abort all remaining steps.
+            </li>
             <li>If any member of <var>presentationUrls</var> is not an <a>a
             priori authenticated URL</a>, then throw a <a>SecurityError</a> and
             abort these steps.