An unknown role is considered a bug, should never occur

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

 /*
  * Copyright (C) 2012, Google 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 657 matching lines @@
 
   if (isHTMLHRElement(*GetNode()))
     return kSplitterRole;
 
   if (IsFieldset())
     return kGroupRole;
 
   return kUnknownRole;
 }
 
+// Determine an accessibility role.

[dmazzoni, 2017/05/24 21:57:02]

This comment doesn't really add much value. :)

Either add more info (like when this is called, whether subclasses override it,
what information is or isn't included in the determination) or delete the
comment
and let the method name speak for itself

[aleventhal, 2017/05/25 17:28:34]

Yeah I'm not sure how that happened, duh. 

 AccessibilityRole AXNodeObject::DetermineAccessibilityRole() {
   if (!GetNode())
     return kUnknownRole;
 
   if ((aria_role_ = DetermineAriaRoleAttribute()) != kUnknownRole)
     return aria_role_;
   if (GetNode()->IsTextNode())
     return kStaticTextRole;
 
   AccessibilityRole role = NativeAccessibilityRoleIgnoringAria();
-  if (role != kUnknownRole)
-    return role;
-  if (GetNode()->IsElementNode()) {
-    Element* element = ToElement(GetNode());
-    // A generic element with tabIndex explicitly set gets GroupRole.
-    // The layout checks for focusability aren't critical here; a false
-    // positive would be harmless.
-    if (element->IsInCanvasSubtree() && element->SupportsFocus())
-      return kGenericContainerRole;
-  }
-  return kUnknownRole;
+  return role == kUnknownRole ? kGenericContainerRole : role;
 }
 
 AccessibilityRole AXNodeObject::DetermineAriaRoleAttribute() const {
   const AtomicString& aria_role =
       GetAOMPropertyOrARIAAttribute(AOMStringProperty::kRole);
   if (aria_role.IsNull() || aria_role.IsEmpty())
     return kUnknownRole;
 
   AccessibilityRole role = AriaRoleToWebCoreRole(aria_role);
 
@@ skipping 2520 matching lines @@
     return String();
   return ToTextControlElement(node)->StrippedPlaceholder();
 }
 
 DEFINE_TRACE(AXNodeObject) {
   visitor->Trace(node_);
   AXObjectImpl::Trace(visitor);
 }
 
 }  // namespace blink