<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.outer {
width: 500px;
height: 500px;
background-color: pink;
}
.box {
width: 50%;
height: 0;
padding-top: 50%;
background-color: orangered;
}
</style>
</head>
<body>
<div class="outer">
<div class="box"></div>
</div>
</body>
</html>