如何设置标题单元格的默认宽度

<table>
  <colgroup>
    <col width="100px">
    <col width="200px">
    <col width="150px">
  </colgroup>
  <thead>
    <tr>
      <th>标题1</th>
      <th>标题2</th>
      <th>标题3</th>
    </tr>
  </thead>
  <tbody>
    <!-- 表格内容 -->
  </tbody>
</table>

<style>
  th {
    width: 100px;
  }
</style>
<table>
  <thead>
    <tr>
      <th>标题1</th>
      <th>标题2</th>
      <th>标题3</th>
    </tr>
  </thead>
  <tbody>
    <!-- 表格内容 -->
  </tbody>
</table>

<script>
  window.onload = function() {
    var thElements = document.getElementsByTagName('th');
    for (var i = 0; i < thElements.length; i++) {
      thElements[i].style.width = '100px';
    }
  };
</script>
<table>
  <thead>
    <tr>
      <th>标题1</th>
      <th>标题2</th>
      <th>标题3</th>
    </tr>
  </thead>
  <tbody>
    <!-- 表格内容 -->
  </tbody>
</table>