如何完成一个表格表头不动tbody区域用滚动条可以滚动

<table width="300"  border="0" cellpadding="0" cellspacing="1" bgcolor="#0099CC">
  <tr>
    <th width="25%" nowrap><font color="#FFFFFF"> 123 </font></th>
    <th width="25%" nowrap><font color="#FFFFFF"> 456 </font></th>
    <th width="25%" nowrap><font color="#FFFFFF"> 789 </font></th>
    <th width="25%" nowrap><font color="#FFFFFF"> abc </font></th>
  </tr>
</table>
<span style="overflow-y:auto;height:100px;width:320px">
<table width="300"  border="0" cellpadding="0" cellspacing="1" bgcolor="#0099CC">
  <tr bgcolor="#FFFFFF">
    <td width="25%"> 1</td>
               
    <td width="25%"> 4</td>
               
    <td width="25%"> 7</td>
               
    <td width="25%"> a</td>
        </tr>
       
  <tr bgcolor="#FFFFFF">
    <td > 2</td>
               
    <td > 5</td>
               
    <td> 8</td>
               
    <td> b</td>
        </tr>
       
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
  <tr bgcolor="#FFFFFF">
    <td> 3 </td>
               
    <td> 6 </td>
               
    <td> 9 </td>
               
    <td > c </td>
        </tr>
</table>
</span>

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