Add UA backdrop style.

third_party/WebKit/Source/core/css/html.css

 /*
  * The default style sheet used to render HTML.
  *
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 1063 matching lines @@
 }
 
 bdo {
     unicode-bidi: bidi-override;
 }
 
 textarea[dir=auto i] {
     unicode-bidi: -webkit-plaintext;
 }
 
+::backdrop {
+  display: block !important;
+}
+
 dialog:not([open]) {
     display: none
 }
 
 dialog {
     position: absolute;
     left: 0;
     right: 0;
     width: -webkit-fit-content;
     height: -webkit-fit-content;
@@ skipping 24 matching lines @@
 }
 
 /* Disable multicol in printing, since it's not implemented properly. See crbug.com/99358 */
 
 @media print {
     * { -webkit-columns: auto !important; }
 }
 
 /* noscript is handled internally, as it depends on settings. */