如何使用Markdown在表格的左上角单元格中绘制对角线？

<style>
  .diagonal-table {
    border-collapse: collapse;
  }
  .diagonal-table th,
  .diagonal-table td {
    border: 1px solid black;
    padding: 8px;
    text-align: center;
  }
  .diagonal-table th.diagonal {
    position: relative;
  }
  .diagonal-table th.diagonal::before,
  .diagonal-table th.diagonal::after {
    content: "";
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    border: 1px solid black;
  }
  .diagonal-table th.diagonal::before {
    border-bottom: none;
    border-right: none;
  }
  .diagonal-table th.diagonal::after {
    border-top: none;
    border-left: none;
  }
</style>

<table class="diagonal-table">
  <tr>
    <th class="diagonal"></th>
    <th>Header 1</th>
    <th>Header 2</th>
  </tr>
  <tr>
    <th>Header 1</th>
    <td>Data 1-1</td>
    <td>Data 1-2</td>
  </tr>
  <tr>
    <th>Header 2</th>
    <td>Data 2-1</td>
    <td>Data 2-2</td>
  </tr>
</table>