HTML左中右 布局为什么不能居中
<!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=gb2312" />
<title>左中右</title>
</head>
<style type="text/css">
.mian{
margin:10px auto;
}
#top{
height:600px;
width:200px;
background-color:#00FF00;
float:left;
}
#zhong{
height:600px;
width:500px;
margin:0px 10px;
background-color:#003399;
float:left;
}
#bottom{
height:600px;
width:200px;
background-color:#00FF00;
float:left;
}
</style>
<body>
<div class="mian">
<div id="top"></div>
<div id="zhong"></div>
<div id="bottom"></div>
</div>
</body>
</html>