<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
.a1{
width: 500px;
height: 500px;
background-color: red;
}
.a2{
width: 200px;
height: 200px;
background-color: green;
position: relative;
left: 50%;
top: 50%;
-webkit-transform: translate(-50%,-50%);
-ms-transform: translate(-50%,-50%);
transform: translate(-50%,-50%);
}
</style>
</head>
<body>
<div class="a1">
<div class="a2"></div>
</div>
</body>
</html>
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