Use border-box sizing for the HR element.

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 122 matching lines @@
     text-align: -webkit-center
 }
 
 hr {
     display: block;
     -webkit-margin-before: 0.5em;
     -webkit-margin-after: 0.5em;
     -webkit-margin-start: auto;
     -webkit-margin-end: auto;
     border-style: inset;
-    border-width: 1px
+    border-width: 1px;
+    box-sizing: border-box
 }
 
 map {
     display: inline
 }
 
 video {
     object-fit: contain;
 }
 
@@ skipping 956 matching lines @@
 @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. */