塊級(jí)元素固定寬度，水平居中
代碼如下：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>demo</title>
<style>
#layout {
background:#CCC;
margin:0 auto;
padding:10px;
height:350px;
}
#content {
width:300px;
height:200px;
margin-left:auto;
margin-right:auto;
border:1px transparent;
background:red;
}
</style>
</head>
<body>
<div id = "layout">
<div id = "content">寬300px,高200px,在頁面水平居中</div>
</div>
</body>
</html>
