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"guidelines for selecting and defining key values" is a dead link in "6.5 Keyboard Event key Value Tables". https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#key-value-tables And the spec says, "Implementations MAY support additional key values", but I strongly believe that they MUST NOT support new key names without discuss in W3C.
Broken link has been fixed. I added an "issue" in the spec to track this in our next meeting.
The text indicating that user agents can define their own values has been removed and replaced with a suggestion to file bugs instead.