先思考，在看代码哦。
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
<style>
*{
padding: 0;
margin: 0;
}
html,body{
width: 100%;
height: 100%;
}
.box{
position: fixed;
top: 0;
left: 0;
bottom: 0;
right: 0;
background:rgba(0,0,0,0.5);
}
.son{
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%,-50%);
background: white;
}
</style>
</head>
<body>
<div class="box">
<div class="son">hello world</div>
</div>
</body>
</html>
