Mira, aqui te dejo un ejemplo bien completo, a ver que te parece!
SaludosSsSsSs!!!
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="JavaScript">
function opaco(){
if (document.all)
document.getElementById('celda').style.filter = "alpha(opacity: 30)";
else
document.getElementById('celda').style.MozOpacity = "0.3";
}
</script>
</head>
<body bgcolor="#ffffff" onLoad="opaco()">
<table width="75%" border="0">
<tr>
<td id="celda" bgcolor="orange"> </td>
<td bgcolor="orange"> </td>
</tr>
</table>
</body>
</html>