reason phrases [...] MAY be replaced by local equivalents without affecting the protocol
Could come in handy for debugging?
HTTP applications are not required to understand the meaning of all registered status codes, [...]. However, applications MUST understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class
Interesting edge case. And HTTP status codes are arithmetic types, not enums.
