window.onload

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
<head>
  <meta charset="utf-8" />
  <title>Test</title>
  <script type="application/javascript">
    /* <![CDATA[ */
    window.onload = function(evt) {
      alert('Loaded!');
    }
    /* ]]> */
  </script>
</head>

<body>
<h1>Test</h1>
<p>This is a test</p>
</body>

</html>