<!doctype html>
<style>
col {
border:2px solid green;
}
</style>
<table style="border-collapse:collapse">
<colgroup>
<col span=3 id="target">
</colgroup>
<tbody>
<tr>
<td>a</td>
<td>b</td>
</tr>
</tbody>
</table>
<script>
document.body.offsetTop;
document.querySelector("#target").setAttribute("span", 0);
</script>