弹框居中：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>预约提示</title>
<meta name="keywords" content="">
<meta name="description" content="">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<style>
.shadow-box {
width: 100%;
height: 100%;
position: fixed;
top: 0;
left: 0;
background: rgba(0,0,0,0.3);
}
.shadow-bg {
width: 618px;
height: 400px;
position: absolute;
top: 50%;
left: 50%;
margin-left: -309px;
margin-right: -200px;
background: url(images/pic.png) no-repeat center;
}
</style>
</head>
<body>
<div class="shadow-box">
<div class="shadow-bg">
</div>
</div>
</body>
</html>