<!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>Document</title>
<style>
.box {
width: 600px;
height: 600px;
background: blue;
position: relative
}
.div1 {
/* width: 200px;
height: 200px; */
background: red;
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
margin: auto
}
</style>
</head>
<body>
<div class="box">
<div class="div1"></div>
</div>
</body>
</html>
left:0;right:0;top:0;bottom:0;再加上margin：auto，會使div1居中，如果div1,沒有寬高，則會繼承父集的寬高。適合body 內的遮罩，
