<!DOCTYPE html>
<html>
<head>
<style>
table { background: white }
col[span] { background: green }
td { color: white }
</style>
</head>
<body onload="runTest()">
 <table>
   <colgroup id="x">
     <col span="2"></col>
     <col id="y"></col>
   </colgroup>
   <tr>
     <td>One</td>
     <td>Two</td>
     <td>Three</td>
     <td>Four</td>
   </tr>
 </table>

 <script>
   function runTest() {
     document.body.offsetWidth;
     document.getElementById("x").insertBefore(document.createElement("col"),
                                               document.getElementById("y"));
   }
 </script>
</body>
</html>