rop,
#dialog-parent > #dialog ::backdrop {
    background: red;
}

#dialog-parent > ::backdrop {
    top: 100px;
    left: 100px;
    background: green;
}

#backdrop-ancestor ::backdrop {
    top: 100px;
    left: 300px;
    background: green;
}
</style>
</head>
<body>
Test ::backdrop used in descendant selectors. The test passes if there are two green boxes and no red.

<div id="dialog-parent">
    <dialog id="dialog"></dialog>
</div>
<div id="backdrop-ancestor">
    <p><span><dialog></dialog></span></p>
<