Cute, I like the way ASCII art is preserved.

Small bug: RFCs < 1000 that are not prettifiable have a extraneous zero in the datatracker.ietf.org link.

Eg: http://pretty-rfc.herokuapp.com/RFC123

redirects to: http://pretty-rfc.herokuapp.com/RFC0123 (left padding is probably a mistake here; some day we will have >9999 RFCs)

links to: http://datatracker.ietf.org/doc/rfc0123/ (404)

should link to: http://datatracker.ietf.org/doc/rfc123/

ps. XML format for RFCs that this leans on: http://pretty-rfc.herokuapp.com/RFC2629