HTML5--页面自动居中

注意：
　　margin:0 auto;/**0：上下 auto：左右**/
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
        *{
            margin:0;
            padding:0;
        }
        #wrap{
            width:800px;
            margin:0 auto;
            border:3px solid yellow;
        }
        #header{
            width:100%;
            border:3px solid red;
        }
        #main{
            width:100%;
            border:3px solid green;
        }
        #footer{
            width:100%;
            border:3px solid grey;
        }
    </style>
</head>
<body>
<div id="wrap">
    <div id="header">
        <div>this is header</div>
    </div>
    <div id="main">
        <div>this is main</div>
        <div>red</div>
        <div>red</div>
    </div>
    <div id="footer">
        <div>this is footer</div>
    </div>
</div>
</body>
</html>