title: why the javascript code doesn't work?
body: <pre><code>&lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
  &lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
  &lt;head&gt;
  &lt;meta http-equiv="Content-Type" content="text/html; charset=utf-8" /&gt;
  &lt;title&gt;test&lt;/title&gt;
      &lt;script type="text/javascript"&gt;
      window.onload = function(){
          document.getElementById("myMessage").innerHTML = "hello,world!";
      }
  &lt;/script&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
      &lt;h1 id=="myMessage"&gt;&lt;/h1&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>when i run the file, there is no <code>hello,world!</code> output. why?</p>