overflow:scroll; in <td>

Why does the CSS property overflow:scroll; not work in <td>, while overflow:hidden; works well?

<table border="1" style="table-layout:fixed; width:100px">
  <tr>
    <td style="overflow:scroll; width:50px;">10000000000000000000000000000000000</td>
    <td>200</td>
    <td>300</td>
  </tr>
</table>

From the CSS specs^1,2, I can't see why.