<!DOCTYPE html>
<html>
<head>
<title>index</title>
</head>
<body>
<div class="container">
<div class="box"></div>
</div>
<style type="text/css">
.container{
border: 1px solid #000;
300px;
height: 200px;
position: absolute;
}
.box{
border: 1px solid #000;
50px;
height: 50px;
position: absolute;
margin: auto;
left: 0;
right: 0;
top: 0;
bottom: 0;
}
</style>
</body>
</html>