Move default style for <dialog> to html.css

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 1039 matching lines @@
 }
 
 bdo {
     unicode-bidi: bidi-override;
 }
 
 textarea[dir=auto i] {
     unicode-bidi: -webkit-plaintext;
 }
 
+dialog:not([open]) {
+    display: none
+}
+
+dialog {
+    position: absolute;
+    left: 0;
+    right: 0;
+    width: -webkit-fit-content;
+    height: -webkit-fit-content;
+    margin: auto;
+    border: solid;
+    padding: 1em;
+    background: white;
+    color: black
+}
+
+dialog::backdrop {
+    position: fixed;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    left: 0;
+    background: rgba(0,0,0,0.1)
+}
+
 /* page */
 
 @page {
     /* FIXME: Define the right default values for page properties. */
     size: auto;
     margin: auto;
     padding: 0px;
     border-width: 0px;
 }
 
 /* noscript is handled internally, as it depends on settings. */