AX_ROLE_HORIZONTAL_RULE is not required.

content/renderer/accessibility/blink_ax_tree_source.cc

 // Copyright 2014 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/renderer/accessibility/blink_ax_tree_source.h"
 
 #include <set>
 
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
@@ skipping 54 matching lines @@
     case ui::AX_ROLE_ARTICLE:
       return "article";
     case ui::AX_ROLE_BANNER:
       return "banner";
     case ui::AX_ROLE_BUTTON:
       return "button";
     case ui::AX_ROLE_COMPLEMENTARY:
       return "complementary";
     case ui::AX_ROLE_FOOTER:
       return "contentinfo";
-    case ui::AX_ROLE_HORIZONTAL_RULE:
-      return "separator";
     case ui::AX_ROLE_IMAGE:
       return "img";
     case ui::AX_ROLE_MAIN:
       return "main";
     case ui::AX_ROLE_NAVIGATION:
       return "navigation";
     case ui::AX_ROLE_RADIO_BUTTON:
       return "radio";
     case ui::AX_ROLE_REGION:
       return "region";
@@ skipping 462 matching lines @@
     AddIntListAttributeFromWebObjects(ui::AX_ATTR_OWNS_IDS, owns, dst);
 }
 
 blink::WebDocument BlinkAXTreeSource::GetMainDocument() const {
   if (render_frame_ && render_frame_->GetWebFrame())
     return render_frame_->GetWebFrame()->document();
   return WebDocument();
 }
 
 }  // namespace content