絕對定位 + margin，讓明確寬高的盒子水平垂直居中於窗口
<!DOCTYPE>
<html>
<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">
<style>
/* 讓明確寬高的盒子垂直水平居中於父元素 */
.wangyi-loading{
width: 360px;
height: 360px;
margin:auto;
position: absolute;
top:0;
right:0;
bottom:0;
left:0;
border:5px solid red;
}
</style>
</head>
<body>
<div class="wangyi-loading">
</div>
</body>
</html>
