                        課程
                    
                        /前端開發
                        
                            /HTML/CSS
                        
                        /企業網站綜合布局實戰

請問這是為什么

<!DOCTYPE html>
<html>
<head>
?<meta charset="UTF-8">
?<title>企業網站</title>
<link rel="stylesheet" type="text/css" href="css/mycss.css">
</head>
<body>
<div>
<div>
?<ul>
??<li>聯系我們</li>
??<li>加入收藏</li>
??<li> 設為主頁</li>
?</ul>
?</div>
</div>
<div>
??? <div>
??? ?<div> <img src="image/logo.jpg"></div>
??? ?<div><img src="image/tel.jpg">24小時服務熱線：<span?>123-456-7890</span></div>
??? </div> 
??? <div>
??? ?<div></div>
??? ?<div>
??? ???? <div>
??? ?????? <ul>
??? ???? <li>首頁</li>
??? ???? <li>關于慕課</li>
??? ???? <li>新聞動態</li>
??? ???? <li>課程中心</li>
??? ???? <li>在線課程</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;

}

.nav-right{
?height:40px;
?width: 10px;
?float: left;
?background: url(../image/nav_right.jpg) no-repeat;
}
.nav-mid-left{
?float: left;
?width: 680px;

}
.nav-mid-right{
?float: left;
?width: 300px;

}
.nav-mid-left li{
?float: left;
??? list-style-type: none;
}