<table width="100%" bgcolor="green">
<tr>
<td id="wtd" width="50%" style="border:red 5px groove" onclick="wchg();">
test
</td>
<td width="50%" style="border:yellow 5px groove">
test2
</td>
</tr>
</table>
<script>
function wchg(){
document.getElementById('wtd').style.width = '10%';
}
</script>