<!DOCTYPE html>
<style>
dialog {
    padding: 0px;
    border: none;
    margin: 0px;
}

#bottom::backdrop {
    top: 100px;
    left: 100px;
    height: 300px;
    width: 300px;
    background-color: rgb(0, 50, 0);
    z-index: 100;  /* z-index has no effect. */
}

#bottom {
    top: 125px;
    left: 125px;
    height: 250px;
    width: 250px;
    background-color: rgb(0, 90, 0);
}

#middle::backdrop {
    top: 150px;
    left: 150px;
    height: 200px;
    width: 200px;
    background-color: rgb(0, 130, 0);
    z-index: -100;  /* z-index has no effect. */
}

#middle {
    top: 175px;
    left: 175px;
    height: 150px;
    width: 150px;
    background-color: rgb(0, 170, 0);
}

#top::backdrop {
    top: 200px;
    left: 200px;
    height: 100px;
    width: 100px;
    background-color: rgb(0, 210, 0);
    z-index: 0;  /* z-index has no effect. */
}

#top {
    top: 225px;
    left: 225px;
    height: 50px;
    width: 50px;
    background-color: rgb(0, 255, 0);
    z-index: -1000;  /* z-index has no effect. */
}
</style>
<body>
Test for dialog::backdrop stacking order. The test passes if there are 6
boxes enclosed in each other, becoming increasingly smaller and brighter
green.
<dialog id="top"></dialog>
<dialog id="middle"></dialog>
<dialog id="bottom"></dialog>
<script>
var topDialog = document.getElementById('top');
var middleDialog = document.getElementById('middle');
var bottomDialog = document.getElementById('bottom');
topDialog.showModal();
bottomDialog.showModal();
topDialog.close();  // Just to shuffle the top layer order around a little.
middleDialog.showModal();
topDialog.showModal();
</script>
</body>