<table>
  <thead>
    <tr>
      <th>見出し1</th>
      <th>見出し2</th>
      <th>見出し3</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>内容1</td>
      <td>内容2</td>
      <td class="problem">ーーーーーー!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!</td>
    </tr>
  </tbody>
</table>
<p class="w400">
  これが本来の400px
</p>
body {
  word-wrap: break-word;
  overflow-wrap : break-word;
}
table {
  border-collapse:collapse; 
  border: 1px solid #000000;
  width: 400px;
  table-layout: fixed;
}
td {
  border: 1px solid #000000;
}
/*  400pxの目安 */
p.w400 {
  border: 1px solid #ff0000;
  width: 400px;
}

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