Use display:block for dialog in UA stylesheet.

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 1061 matching lines @@
 
 textarea[dir=auto i] {
     unicode-bidi: -webkit-plaintext;
 }
 
 dialog:not([open]) {
     display: none
 }
 
 dialog {
+    display: block;
     position: absolute;
     left: 0;
     right: 0;
     width: -webkit-fit-content;
     height: -webkit-fit-content;
     margin: auto;
     border: solid;
     padding: 1em;
     background: white;
     color: black
@@ skipping 21 matching lines @@
     padding: 0px;
     border-width: 0px;
 }
 
 /* Allows thead sections to print at the top of each page. */
 @media print {
     thead { break-inside:avoid; }
 }
 
 /* noscript is handled internally, as it depends on settings. */