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                        /如何用CSS進行網頁布局

footer加入clear:both后，當瀏覽器寬度小于960px時，為什么footer右側還是會露出body背景色？如何解決？

<!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>混合布局</title>
<style>
body{?margin:0;?padding:0;?font-size:30px;?font-weight:bold;background-color:?red;}
div{?text-align:center;?line-height:50px}
.top{?height:100px;background:#9CF}
.head,.main{?width:960px;?/*【任務1】*/margin:0?auto;}
.head{?height:100px;?background:#F90}
.left{?width:220px;?height:600px;?background:#ccc;?/*【任務2】*/float:left;}
.right{?width:740px;?height:600px;background:#FCC;?float:right}
.r_sub_left{?width:540px;?height:600px;?background:#9C3;?float:left}
.r_sub_right{?width:200px;?height:600px;?background:#9FC;?/*【任務3】*/float:right;}
.footer{?height:50px;?background:#9F9;?/*【任務4】*/clear:both;}
</style>
</head>

<body>
<div?class="top">
????<div?class="head">head</div>
</div>
<div?class="main">
????<div?class="left">left</div>
????<div?class="right">
????????<div?class="r_sub_left">sub_left
????????</div>
????????<div?class="?r_sub_right">sub_right
????????</div>
????</div>
</div>
<div?class="footer">footer</div>
</body>
</html>

設置body背景色為紅色后，調小寬度可看到右下角的紅色。