console 命令行工具 X clear
console
<div id="container" style="width:600px">

<div id="header" style="background-color:#FFA500;">
<h1 style="margin-bottom:0;text-align:center;">2.28上机课练习</h1></div>

<div id="menu" style="background-color:#FFD700;height:500px;width100px;float:left;">
  <dl>
<dt><b><center>学习内容</center></b></dt><br>
<dd>HTML</dd><br>
<dd>CSS</dd><br>
<dd>JavaScript</dd><br>
  </dl>
    </div>

<div id="content" style="background-color:#EEEEEE;height:500px;width:420px;float:left;">
上机练习</div>

<div id="footer" style="background-color:#FFA500;clear:both;text-align:center;">
版权 © runoob.com</div>

</div>