prova così
codice:
<body> 

<script>
var larghezza=400;	
var left=screen.availWidth/2-larghezza/2;
document.write('<table style="position:absolute;top:10;left:'+left+'" border="1" width="'+larghezza+'">');
</script>
  <tr>
    <td width="50%"></td>
    <td width="50%">100</td>
  </tr>
  <tr>
    <td width="50%"></td>
    <td width="50%">100</td>
  </tr>
</table>  
  
<script>
var larghezza=400;	
var left=screen.availWidth/2-larghezza/2;
document.write('<table style="position:absolute;top:10;left:'+left+'" border="1" width="'+larghezza+'">');
</script>
  <tr>
    <td width="50%">50</td>
    <td width="50%"></td>
  </tr>
  <tr>
    <td width="50%">50</td>
    <td width="50%"></td>
  </tr>
</table>

</body>