使用 css 在 上设置强制宽度，该宽度位于 下，使用 colspan 而不使用 colgroup

.sample {
  width: 100%;
  table-layout: fixed;
}

.sample td:nth-child(1),
.sample th:nth-child(1) {
  width: 30%;
}

.sample td:nth-child(2),
.sample th:nth-child(2) {
  width: 70%;
}

<table class="sample" border="1">
  <thead>
    <tr>
      <th colspan="2">Header</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>30% This columns has a lot of content and a really long woooooooooooooooooooooooooooooooooooooord which is not wrapable by the browser by default. This column still should be 30%with.</td>
      <td>70%</td>
    </tr>
  </tbody>
</table>