I have reviewed this document as part of the Ops area directorate's ongoing effort to review all IETF documents being processed by the IESG.  These comments were written primarily for the benefit of the Ops area directors.
Document editors and WG chairs should treat these comments just like any other last-call comments.

The document specifies an extension to the CBOR diagnostic notation to include application-oriented literals for representing date/time and IP addresses/prefixes. It also defines some additional syntax for handling unknown application-extension identifiers and elisions (omissions) in the notation.
The document is clear and comprehensive. 
While the document specifies using tag 999 to represent unknown application-extension identifiers, it does not provide strict rules on how an EDN consumer should handle such identifiers beyond preserving the information. It would be nice to have more specific error handling guidance. 

Best regards, Linda Dunbar