<!DOCTYPE html> <html> <head> <style> table { background: white } col { background: green } td { color: white } </style> </head> <body onload="runTest()"> <table> <col id="x" span="3"> <tr> <td>One</td> <td>Two</td> <td>Three</td> </tr> </table> <script> function runTest() { document.body.offsetWidth; document.getElementById("x").setAttribute("span", 2); } </script> </body> </html>