简单的javasript问题。

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>无标题文档</title>
<script language="JavaScript" type="text/JavaScript">
<!--
function MM_reloadPage(init) {  //reloads the window if Nav4 resized
  if (init==true) with (navigator) {if ((appName=="Netscape")&&(parseInt(appVersion)==4)) {
    document.MM_pgW=innerWidth; document.MM_pgH=innerHeight; onresize=MM_reloadPage; }}
  else if (innerWidth!=document.MM_pgW || innerHeight!=document.MM_pgH) location.reload();
}
MM_reloadPage(true);
//-->
</script>
</head><body>
<div id="Layer1" style="position:absolute; width:195px; height:100px; z-index:1; left: 148px; top: 17px;" >
  <table width="197" height="58" border="0" cellspacing="1" bordercolor="#FFFFFF" bgcolor="#999999"  id="Purview2"  style="DISPLAY: none" onMouseMove="aa()"  onMouseOut="bb()">
    <tr>
      <td bgcolor="#CCCCCC"><a href="http://163.com">aaaaa</a></td>
    </tr>
    <tr>
      <td bgcolor="#CCCCCC"><a href="http://163.com">abbbaaaa</a></td>
    </tr>
  </table>
</div>
<table bgcolor="#999999">
<tr>
<td   onMouseMove="aa()"  onMouseOut="bb()">aaaaaaaaaaaaaaaaaaa</td>
</tr>
</table>
</body>
</html>
<script>
function aa()
{
     Purview2.style.display = "";
}
function bb()
{
     Purview2.style.display = "none";
}
</script>