请问top层的高度是多少，怎么得到的

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>固定层</title>
<style type="text/css">
*{margin:0; padding:0; font-size:12px; list-style:none;}
a{text-decoration:none;}
a:hover{text-decoration:underline;}
a img{border:none;}
body{background:#efefec;}
#top{width:100%; 
overflow:hidden;
border-bottom:1px solid #dadad9;
background:#fff; 
float:left; 
position:fixed; 
top:0;
left:0;
text-align:center;}
#top input{border:none;}

.left{float:left; margin-left:10px;}
.search{float:left; margin-left:10px; margin-top:7px;}
.select_btn{background:url(images/select_top.gif) no-repeat; width:57px; height:42px; cursor:pointer; float:left;}
.search_input{background:url(images/input_search.gif) no-repeat; width:104px; height:28px; float:left; padding-left:5px;}
.search_btn{background:url(images/search_btn.gif) no-repeat; width:29px; height:28px; float:left;}

.right{float:right; margin-right:10px; padding:5px 10px 0; border-left:1px solid #f5f5f5; border-right:1px solid #f5f5f5; height:37px;}
.topbtn{background:url(images/top_btn.gif) no-repeat; width:54px; height:30px; line-height:30px; color:#444;}

.margin_l_10{margin-left:10px;}
.margin_t_5{margin-top:5px;}
.center{margin:0 15px;}
.center_r{margin-right:15px;}

#mainbody{width:80%; 
overflow:hidden; 
margin:63px auto 0;
}
.content{width:236px; border:1px solid #dcdcda; float:left;}
.content li{margin-bottom:15px; text-align:center;}
.content li h2{text-align:left; padding:12px 15px; font-weight:normal; color:#444; background:#fff; margin-top:-3px; background:#fafafa;}
.content li dl{border-top:1px solid #f2f2f2; text-align:left;}
.content li dt{padding:15px; float:left; border-right:1px solid #f2f2f2;}
.content li dd{padding-top:18px; text-indent:15px;}
.content li dd a{color:#9e7e6b;}

#footer{width:100%; background:#fff; text-align:center; padding:20px 0; color:#999; border-top:#ccc; margin-top:20px;}
</style>
</head>

<body>
<div id="top">
  <div class="left">
    <div class="select_btn"></div>
    <div class="search">
      <input type="text" class="search_input" />
      <input type="button" class="search_btn" />
    </div>
  </div>
  <a href="#" class="margin_t_5"><img src="images/logo.gif" width="80" height="35" /></a>
<div class="right">
    <input type="button" value="登录" class="topbtn" />
    <input type="button" value="注册" class="topbtn margin_l_10" />
  </div>
</div>
<div id="mainbody">
  <div class="content">
    <ul>
      <li><img src="images/img_03.jpg" width="204" height="331" />
        <h2>人潮</h2>
        <dl>
          <dt><img src="images/head_03.jpg" width="34" height="34" /></dt>
          <dd><a href="#">霹雳红唇</a> 采集到 <a href="#">时光</a></dd>
        </dl>
      </li>
      <li><img src="images/img_04.jpg" width="236" height="303" />
        <h2>熊熊与蘑菇女</h2>
        <dl>
          <dt><img src="images/head_02.jpg" width="34" height="34" /></dt>
          <dd><a href="#">白雪公主</a> 采集到 <a href="#">漫漫画语</a></dd>
        </dl>
      </li>
      <li><img src="images/img_08.jpg" width="236" height="333" />
        <h2>幻化</h2>
        <dl>
          <dt><img src="images/head_01.jpg" width="34" height="34" /></dt>
          <dd><a href="#">流浪猫</a> 采集到 <a href="#">漫漫画语</a></dd>
        </dl>
      </li>
    </ul>
  </div>
  <div class="content center">
       <ul>
      <li><img src="images/img_05.jpg" width="236" height="342" />
        <h2>桃花满枝</h2>
        <dl>
          <dt><img src="images/head_02.jpg" width="34" height="34" /></dt>
          <dd><a href="#">白雪公主</a> 采集到 <a href="#">淡彩</a></dd>
        </dl>
      </li>
      <li><img src="images/img_02.jpg" width="236" height="243" />
        <h2>小矮人</h2>
        <dl>
          <dt><img src="images/head_04.jpg" width="34" height="34" /></dt>
          <dd><a href="#">小p孩</a> 采集到 <a href="#">漫漫画语</a></dd>
        </dl>
      </li>
      <li><img src="images/img_07.jpg" width="236" height="330" />
        <h2>古代美女</h2>
        <dl>
          <dt><img src="images/head_04.jpg" width="34" height="34" /></dt>
          <dd><a href="#">小p孩</a> 采集到 <a href="#">倾国倾城</a></dd>
        </dl>
      </li>
    </ul>
  </div>
  <div class="content center_r">
       <ul>
      <li><img src="images/img_06.jpg" width="236" height="373" />
        <h2>阿狸爱</h2>
        <dl>
          <dt><img src="images/head_02.jpg" width="34" height="34" /></dt>
          <dd><a href="#">白雪公主</a> 采集到 <a href="#">爱漫</a></dd>
        </dl>
      </li>
      <li><img src="images/img_08.jpg" width="236" height="333" />
        <h2>幻化</h2>
        <dl>
          <dt><img src="images/head_01.jpg" width="34" height="34" /></dt>
          <dd><a href="#">流浪猫</a> 采集到 <a href="#">漫漫画语</a></dd>
        </dl>
      </li>
      <li><img src="images/img_04.jpg" width="236" height="303" />
        <h2>熊熊与蘑菇女</h2>
        <dl>
          <dt><img src="images/head_02.jpg" width="34" height="34" /></dt>
          <dd><a href="#">白雪公主</a> 采集到 <a href="#">漫漫画语</a></dd>
        </dl>
      </li>
    </ul>
  </div>
  <div class="content">
       <ul>
      <li><img src="images/img_07.jpg" width="236" height="330" />
        <h2>古代美女</h2>
        <dl>
          <dt><img src="images/head_04.jpg" width="34" height="34" /></dt>
          <dd><a href="#">小p孩</a> 采集到 <a href="#">倾国倾城</a></dd>
        </dl>
      </li>
      <li><img src="images/img_01.jpg" width="236" height="416" />
        <h2>Queen&Princess</h2>
        <dl>
          <dt><img src="images/head_02.jpg" width="34" height="34" /></dt>
          <dd><a href="#">白雪公主</a> 采集到 <a href="#">漫漫画语</a></dd>
        </dl>
      </li>
      <li><img src="images/img_03.jpg" width="204" height="331" />
        <h2>人潮</h2>
        <dl>
          <dt><img src="images/head_03.jpg" width="34" height="34" /></dt>
          <dd><a href="#">霹雳红唇</a> 采集到 <a href="#">时光</a></dd>
        </dl>
      </li>
    </ul>
  </div>
</div>
<div id="footer">慕课@版权所有</div>
</body>
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