title: How to pick element inside iframe using document.getElementById
body: <p>I have a <code>iframe</code> like this</p>
  
  <pre><code>&lt;iframe name="myframe1" id="myframe1" width="100%" height="100%" src="a.html"&gt;
  &lt;html&gt;
      &lt;head&gt;&lt;/head&gt;
      &lt;frameset name="myframe2" cols="0%, 100%" border="0" frameBorder="0" frameSpacing="0"&gt;
          &lt;frame name="page1" src="c.html" scrolling="no"&gt;&lt;/frame&gt;
          &lt;frame name="page2" src="d.html" &gt;
              &lt;html&gt;
                  &lt;head&gt;&lt;/head&gt;
                  &lt;body id="top"&gt;
                      &lt;div id="div1"&gt;
                          &lt;div id="div2"&gt;
                              &lt;div id="div3"&gt;
                                  &lt;ul id="x"&gt;
                                      &lt;li&gt;a&lt;/li&gt;
                                      &lt;li&gt;b&lt;/li&gt;
                                  &lt;/ul&gt;
                              &lt;/div&gt;
                          &lt;/div&gt;
                      &lt;/div&gt;
                  &lt;/body&gt;
              &lt;/html&gt;
  
          &lt;/frame&gt;
  
      &lt;/frameset&gt;
  &lt;/html&gt;
  &lt;/iframe&gt;
  </code></pre>
  
  <p>I want to refer to the element "x". I tried in several ways but I couldn't find a solution. </p>