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

/* The default CSS style from the spec gives a background color but no dimensions,
 * so the backdrop has zero size by default.
 * http://www.whatwg.org/specs/web-apps/current-work/multipage/rendering.html#rendering
 */
dialog::backdrop {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
}
</style>
<body>
Test for the default user agent style of dialog::backdrop. The test passes if
there is a green box, above a very lightly translucent gray box spanning the
viewport.
<dialog></dialog>
<script>
document.querySelector('dialog').showModal();
</script>
</body>