第二层的div无法撑开第一层div

Question

table.center,div.center
{
    margin-left:2%;
    margin-right:2%;
    width:96%;
    min-width:240px;
max-width:480px;
max-height:320px;
}div.outLine {
border-style:none;
padding:0px;
margin:0px;
border-collapse:collapse;
min-width:240px;
max-width:480px;
max-height:320px;
}
<div id="FirstLevel" style="border-bottom: 1px solid; border-left: 1px solid; border-top: 1px solid;
        border-right: 1px solid;position:static" class="outLine">        
               
        <div id="SecondLevel" class="content">           
            <table class="center">
                <tbody>
                    
                    <tr class="odd">
                        <td>                                              
                            <div class="tag2">
                                <a href="http://localhost:2891/default.aspx?id=00000000-0000-0000-0000-000000000211&amp;display=summary">
                                    uuuuuuuuuuuuu uuuuuuuuuuuuuuuuu uuuuuuuuuuuuuuuuuuuu uuuuuuuuuuuuu uuuuuuuuuuuuuuuuuuuu
                                    4563456256</a>
                            </div>
                        </td>
                        
                    </tr>
                </tbody>
            </table>           
        </div>       
    </div>

Answer 1

<div id="FirstLevel" style="border-bottom: 1px solid; border-left: 1px solid; border-top: 1px solid;
  border-right: 1px solid;position:static" class="outLine">  
   
  <div id="SecondLevel" class="content">  
  <table class="center">
  <tbody>
   
  <tr class="odd">
  <td>  
  <div class="tag2">
  <a href="http://localhost:2891/default.aspx?id=00000000-0000-0000-0000-000000000211&amp;display=summary">
  uuuuuuuuuuuuu uuuuuuuuuuuuuuuuu uuuuuuuuuuuuuuuuuuuu uuuuuuuuuuuuu uuuuuuuuuuuuuuuuuuuu
  4563456256</a>
  </div>
  </td>
   
  </tr>
  </tbody>
  </table>  
<div class="clear" style="clear:both"></div>
  </div>  
<div class="clear" style="clear:both"></div>
  </div>

Answer 2

为什么要多加两个div？<div class="clear" style="clear:both"></div>
为什么不在原来的div上加clear:both？