mozilla-release/layout/tables/reftests/1220621-1f.html

<!DOCTYPE html>
<style>
  table {
    table-layout: fixed;
    border: 1px solid black;
    width: 300px;
  }
  td {
    background: yellow;
    border: 1px solid purple;
  }
</style>
<table>
  <colgroup>
    <col>
    <col>
    <col>
    <col>
  </colgroup>
  <tbody>
    <td>One</td>
    <td>Two</td>
    <td>Three</td>
  </tbody>
</table>
<script>
  var t = document.querySelector("table");
  // Flush layout
  var width = t.offsetWidth;
  // Remove the one colgroup
  document.querySelector("colgroup").remove();
</script>