<!DOCTYPE html>
<style>
.dialog-default-ua-style {
    position: absolute;
    left: 0;
    right: 0;
    margin: auto;
    border: solid;
    padding: 1em;
    background: white;
    color: black;
}

#dialog {
    height: 100px;
    width: 100px;
    top: 100px;
    z-index: 1000;
    background: green;
}

#backdrop {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0,0,0,0.1);
    z-index: 100;
}
</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.
<div id="backdrop"></div>
<div class="dialog-default-ua-style" id="dialog"></div>
</body>