css怎么合并单元格

<table border="1" style={{margin:200}}>
      <tbody>
        <tr>
          <th colspan="2">我是占位符</th>
          <th colspan="2">我是占位符</th>
        </tr>
        <tr>
          <th rowspan="2">我是占位符</th>
          <th>我是占位符</th>
          <th>我是占位符</th>
          <th>我是占位符</th>
        </tr>
        <tr>
          <th>我是占位符</th>
          <th>我是占位符</th>
          <th>我是占位符</th>
        </tr>
      </tbody>
    </table>

<td >
    <div class="biaoTou">       
    </div>
</td>
.biaoTou {  
  border-top: 200px #199fff solid; /*上边框宽度等于表格第一行行高*/  
  border-left: 200px #ff8838 solid; /*左边框宽度等于表格第一行第一格宽度*/  
}