See https://www.rfc-editor.org/rfc/rfc2324 and https://www.rfc-editor.org/rfc/rfc7168. Then take a look at the dates the RFCs were published :)
The "data" for most coffee URIs contain no caffeine.
Haven't read through that in a long time. Still hoping to get a HTCPCP-compliant coffee pot some day.