<!DOCTYPE html>
<html>
<head>
<style>
dialog {
    height: 150px;
    width: 150px;
}

::backdrop {
    display: none;
}

#bottomDialog {
    background-color: yellow;
    top: 100px;
    z-index: 1000;
}

#middleDialog {
    background-color: blue;
    top: 150px;
    left: 50px;
    z-index: -500;
}

#topDialog {
    background-color: green;
    top: 200px;
    left: 100px;
    z-index: -1000;
}

.red {
    background-color: red;
    top: 250px;
    left: 0px;
}
</style>
</head>
<body>
This tests that top layer elements are stacked correctly even if nested in the DOM tree.
The test passes if you see no red rectangles and see 3 rectangles stacked in the following order (from bottom to top): yellow, blue, green.

<dialog id="topDialog">
    <dialog id="middleDialog">
        <dialog id="bottomDialog">
            <dialog id="hiddenDialog" class="red">
                <dialog id="hiddenDialogChild" class="red"></dialog>
            </dialog>
        </dialog>
    </dialog>
</dialog>
<script>
document.getElementById('hiddenDialogChild').showModal();
document.getElementById('hiddenDialog').showModal();
document.getElementById('bottomDialog').showModal();
document.getElementById('middleDialog').showModal();
document.getElementById('topDialog').showModal();
document.getElementById('hiddenDialog').close();
</script>
</body>
</html>