AX_ROLE_HORIZONTAL_RULE is not required.

content/browser/accessibility/browser_accessibility_win.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/accessibility/browser_accessibility_win.h"
 
 #include <UIAutomationClient.h>
 #include <UIAutomationCoreApi.h>
 
 #include "base/strings/string_number_conversions.h"
@@ skipping 3541 matching lines @@
       break;
     }
     case ui::AX_ROLE_GROW_AREA:
       ia_role_ = ROLE_SYSTEM_GRIP;
       ia_state_ |= STATE_SYSTEM_READONLY;
       break;
     case ui::AX_ROLE_HEADING:
       role_name_ = html_tag;
       ia2_role_ = IA2_ROLE_HEADING;
       break;
-    case ui::AX_ROLE_HORIZONTAL_RULE:
-      ia_role_ = ROLE_SYSTEM_SEPARATOR;
-      break;
     case ui::AX_ROLE_IFRAME:
       ia_role_ = ROLE_SYSTEM_DOCUMENT;
       ia2_role_ = IA2_ROLE_INTERNAL_FRAME;
       ia_state_ = STATE_SYSTEM_READONLY;
       break;
     case ui::AX_ROLE_IMAGE:
       ia_role_ = ROLE_SYSTEM_GRAPHIC;
       ia_state_ |= STATE_SYSTEM_READONLY;
       break;
     case ui::AX_ROLE_IMAGE_MAP:
@@ skipping 283 matching lines @@
   // The role should always be set.
   DCHECK(!role_name_.empty() || ia_role_);
 
   // If we didn't explicitly set the IAccessible2 role, make it the same
   // as the MSAA role.
   if (!ia2_role_)
     ia2_role_ = ia_role_;
 }
 
 }  // namespace content