Editorial nit:
[HEADERS] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004, <https://www.rfc-editor.org/info/rfc3864>.
appears as normative reference, but only occurs in the IANA Considerations. I believe an informative reference is sufficient.
Best regards, Julian