overflow:scroll; in

You have to wrap it in a div, that will work:

<table border="1" style="table-layout:fixed; width:500px">
  <tr>
    <td style="width:100px;"><div style="overflow:scroll; width:100%">10000000000000000000000000000000000</div></td>
    <td>200</td>
    <td>300</td>
  </tr>
</table>