Add a UA style, "display: contents", for <slot>

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 1081 matching lines @@
 
 dialog::backdrop {
     position: fixed;
     top: 0;
     right: 0;
     bottom: 0;
     left: 0;
     background: rgba(0,0,0,0.1)
 }
 
+slot {
+    display: contents;
+}
+
 /* page */
 
 @page {
     /* FIXME: Define the right default values for page properties. */
     size: auto;
     margin: auto;
     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. */