​x
 
<html>
<head>
<script type="text/javascript">
function setBorderCollapse()
{
document.getElementById('myTable').style.borderCollapse="collapse";
}
</script>
</head>
<body>
​
<table border="1" id="myTable">
  <tr>
    <td>100</td>
    <td>200</td>
  </tr>
  <tr>
    <td>300</td>
    <td>400</td>
  </tr>
</table>
<br />
<input type="button" onclick="setBorderCollapse()" value="Collapse border">
​
</body>
</html>
​