<!DOCTYPE html>
<html>
<head>
    <style>
        .container {
            position: absolute;
            width: 300px;
            height: 300px;
            border: 1px solid black;
        }
        
        .box {
            position: absolute;
            width: 200px;
            height: 200px;
        }
        
        .bottom {
            left: 0px;
            top: 0px;
            background-color: orange;
        }

        .top {
            left: 100px;
            top: 100px;
            background-color: green;
            z-index: -1;
            opacity: 0.8;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="bottom box"></div>
        <div class="top box"></div>
    </div>
</body>
</html>