<script type="text/javascript">
function setFixedTableLayout()
document.getElementById('myTable').style.tableLayout="fixed";
<table id="myTable" border="1" width="100%">
<col width="20%"><col width="40%"><col width="40%">
<td>1000000000000000000000000000</td>
<input type="button" onclick="setFixedTableLayout()" value="Set fixed table layout">