                        课程
                    
                        /前端开发
                        
                            /HTML/CSS
                        
                        /企业网站综合布局实战

请问这是为什么

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>企业网站</title>
<link rel="stylesheet" type="text/css" href="css/mycss.css">
</head>
<body>
<div>
<div>
<ul>
  <li>联系我们</li>
  <li>加入收藏</li>
  <li> 设为主页</li>
</ul>
</div>
</div>
<div>
    <div>
    <div> <img src="image/logo.jpg"></div>
    <div><img src="image/tel.jpg">24小时服务热线：<span >123-456-7890</span></div>
    </div> 
    <div>
    <div></div>
    <div>
         <div>
           <ul>
         <li>首页</li>
         <li>关于慕课</li>
         <li>新闻动态</li>
         <li>课程中心</li>
         <li>在线课程</li>
         <li>人才招聘</li>
           </ul>
            </div>
         <div>
      <form action="" method="post">
       <input type="text" >
      </form>
         </div>
        </div>
    <div></div>
    </div>

</div>
</body>
</html>

*{
margin: 0;
padding: 0;
font-size: 12px;
}
body{
background-color: #f5f5f5; }
.top{
width: 100%;
height: 28px;
background: url(../image/top_bg.jpg) repeat-x;
}
.top-content{
width: 1000px;
margin: 0 auto;

}
.top-content li{
list-style-image: url(../image/li_bg.gif);
float:right;
width: 70px;

}
.wrap{
width: 1000px;
margin: 0 auto;

}
.logo{
height: 80px;
background-color: #fff;
}
.logo-left{
width: 200px;
float: left;

}
.logo-right{
    width: 300px;
    height: 28px;
    margin-top: 30px;
float: right;
color: #8e8e8e;
}
.logo-right img{
vertical-align: middle;
margin-right: 10px;
}
.tel{
font-size: 16px;
color: #c00;
}
.nav{
height: 40px;

}
.nav-left{
width: 10px;
background: url(../image/nav_left.jpg) no-repeat;
    height: 40px;
    float: left;
}
.nav-mid{
width: 980px;
background: url(../image/nav_bg.jpg) repeat-x;
float: left;