<!DOCTYPE html>

<html>
<head>
    <style>
        .container {
            position: relative;
            margin: 50px;
            height: 100px;
            width: 100px;
            border: 1px solid black;
        }
        .box {
            position: absolute;
            top: 0;
            left: 0;
            height: 100%;
            width: 100%;
            background-color: gray;
        }
    </style>
</head>
<body>
<div class="container">
    <div class="box"></div>
</div>
</body>
</html>