The 5.6.2. Keyboard Event Key Location section (source) has a table of several keys that have different locations, and a list of all the possible (and known) values for .location for such keys. It also says:

For all other keys not listed in this table, the location attribute MUST always be set to DOM_KEY_LOCATION_STANDARD.

However, the Insert and Delete keys are missing from that table. Those keys are traditionally together with Home/End and Page Up/Down; and they are also available as Numpad0 and NumpadDecimal.

Additionally, both Firefox and Chrome (on Linux) report Insert and Delete with .location = 3 (DOM_KEY_LOCATION_NUMPAD), deviating from the current draft. Thus, I believe the lack of those keys in the UI Events spec was a mistake. (I have not tested on any other system.)

You can test the behavior yourself by disabling NumLock and trying those keys on either https://dvcs.w3.org/hg/d4e/raw-file/tip/key-event-test.html or https://codepen.io/denilsonsa/pen/epmoma