<!DOCTYPE html>
<style>
#dialog {
    position: absolute;
    top: 100px;
    left: 100px;
    height: 100px;
    width: 100px;
    background: green;
}

#dialog-before {
    position: absolute;
    top: 0px;
}

#dialog-after {
    position: absolute;
    bottom: 0px;
}

#dialog-backdrop {
    position: absolute;
    top: 100px;
    left: 300px;
    height: 100px;
    width: 100px;
    background: green;
}
</style>
<body>
Test for a modal dialog with ::before, ::after, and ::backdrop. The test passes
if there are two green boxes, one with the texts "::before" and "::after" in it.
<div id="dialog">
    <div id="dialog-before">::before</div>
    <div id="dialog-after">::after</div>
</div>
<div id="dialog-backdrop"></div>
<script>
document.querySelector('dialog').showModal();
</script>
</body>