why the javascript code doesn't work?
<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>
