An unknown role is considered a bug, should never occur

third_party/WebKit/Source/modules/accessibility/AXObjectImpl.cpp

 /*
  * Copyright (C) 2008, 2009, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 1968 matching lines @@
     case kNoneRole:
     case kParagraphRole:
     case kPreRole:
     case kPresentationalRole:
     // Spec says we should always expose the name on rows,
     // but for performance reasons we only do it
     // if the row might receive focus
     case kRowRole:
     case kRubyRole:
     case kRulerRole:
+      result = recursive || (CanReceiveAccessibilityFocus() && !IsEditable());
+      break;
+
     case kUnknownRole:
     case kNumRoles:
-      result = recursive || (CanReceiveAccessibilityFocus() && !IsEditable());
+      LOG(ERROR) << "kUnknownRole for " << GetNode();
+      NOTREACHED();
       break;
   }
 
   return result;
 }
 
 AccessibilityRole AXObjectImpl::ButtonRoleType() const {
   // If aria-pressed is present, then it should be exposed as a toggle button.
   // http://www.w3.org/TR/wai-aria/states_and_properties#aria-pressed
   if (AriaPressedIsPresent())
@@ skipping 20 matching lines @@
 }
 
 DEFINE_TRACE(AXObjectImpl) {
   visitor->Trace(children_);
   visitor->Trace(parent_);
   visitor->Trace(cached_live_region_root_);
   visitor->Trace(ax_object_cache_);
 }
 
 }  // namespace blink