<!DOCTYPE html>
<html>
<head>
    <style>
        dialog {
            position: absolute;
            top: 10px;
            left: 10px;
            width: 300px;
            height: 200px;
        }

        .box {
            position: absolute;
            z-index: 0;
            width: 100px;
            height: 100px;
            background-color: green;
        }
        
        .negative {
            margin: 50px;
            z-index: -1;
            background-color: orange;
        }
    </style>
</head>
<body>
    <dialog id="theDialog">
        <div class="box"></div>
        <div class="negative box"></div>
    </dialog>
<script>
    document.getElementById('theDialog').showModal();
</script>
</body>
</html>