这里我要说一下css外容器margin的问题！
先看看下面的例子，
<!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>Demo-1</title>
<style type="text/css">
#wrapper{border:1px #999 solid; width:600px; margin-bottom:20px;}
.left{width:300px; float:left; height:100px; background:red;}
.right{width:300px; float:left; height:300px; background:#ccc;}
#footer{clear:both;width:600px;height:20px; background:#000;border:1px solid #ccc;}
</style>
</head>
<body>
<div id="wrapper">
<div class="left"></div>
<div class="right"></div>
</div>
<div id="footer"></div>
</body>
</html>