KeyboardEvent - Web APIs
they are accessed as keyboardevent.dom_key_location_standard and so forth.
... keyboard location identifiers constant value description dom_key_location_standard 0x00 the key described by the event is not identified as being located in a particular area of the keyboard; it is not located on the numeric keypad (unless it's the numlock key), and for keys that are duplicated on the left and right sides of the keyboard, the key is, for whatever reason, not to be associated with that location.
...the numlock key does not fall into this group and is always encoded with the location dom_key_location_standard.

KeyboardEvent.location - Web APIs
possible values are: constant value description dom_key_location_standard 0 the key has only one version, or can't be distinguished between the left and right versions of the key, and was not pressed on the numeric keypad or a key that is considered to be part of the keypad.
... note: numlock key's key events indicate dom_key_location_standard both on gecko and internet explorer.

nsIDOMWindowUtils
deprecated since gecko 38.0 key_flag_not_synthesized_for_tests 0x0002 key_flag_location_standard 0x0010 location attrubute of the sending key event by sendkeyevent() returns keyboardevent.dom_key_location_standard if this is specified to the aadditionalflags.