<table>
  <tr>
    <th>見出し1</th>
    <td colspan="3">内容</td>
  </tr>
  <tr>
    <th>見出し2</th>
    <td>内容</td>
    <th>見出し3</th>
    <td>内容</td>
  </tr>
</table>

* {
  box-sizing: border-box;
}
body {
  min-height: 100vh;
  margin: 0;
  padding: 20px;
  display: flex;
  justify-content: center;
  align-items: center;
}
table {
    width: 100%;
    border-collapse: collapse;
}
table th,
table td {
    padding: 10px;
    border: 1px solid #ccc;
}
table th {
    background: #f5f5f5;
}

table {
  table-layout: fixed;
}
table th {
  width: 100px;
}