<style> table{margin:0 auto;border:1px solid #000000;border-collapse:collapse;}th,td{border: 1px solid #000000;}   /* 画 实细线*/
 th {background-color: #007bff;   color: #fff;  font-weight: bold;  text-align: center;  padding: 10px;  }       /*表头 样式*/
 tr:nth-child(odd) { background-color: #f2f2f2;} tr:nth-child(even) {background-color: #ffffff;}           /*  正文 隔行 变色*/
</style> 

<table  width=300px; >
 <tr> <th>列1</th>   <th>列2</th> <th>列3</th> </tr> /*表头*/
 <tr> <td>a1</td>    <td>a2</td>  <td>a3</td>  </tr>
 <tr> <td>a1</td>    <td>a2</td>  <td>a3</td>  </tr>
 <tr> <td>a1</td>    <td>a2</td>  <td>a3</td> </tr>
 <tr> <td>a1</td>    <td>a2</td>  <td>a3</td> </tr>
<table>