Tryit Editor v1.4

<html>
<head>
<script type="text/javascript">
function setColSpan()
{
var x=document.getElementById('myTable').rows[0].cells;
x[0].colSpan="2";
x[1].colSpan="6";
}
</script>
</head>

<body>
<table id="myTable" border="1">
<tr>
<td colspan="4">cell 1</td>
<td colspan="4">cell 2</td>
</tr>
<tr>
<td>cell 3</td>
<td>cell 4</td>
<td>cell 5</td>
<td>cell 6</td>
<td>cell 7</td>
<td>cell 8</td>
<td>cell 9</td>
<td>cell 10</td>
</tr>
</table>
<form>
<input type="button" onclick="setColSpan()" value="Change colspan">
</form>
</body>

</html>

Your Result:

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