HTTP Status Codes
The Wikipedia list of HTTP Status Codes is a good resource, listing the ~75 or so codes – both standards and “well known” usages.
With one byte, the 16x16 grid has lots of room to have semantics that are richer than those in HTTP.
HTTP Status Codes were defined as a section of the HTTP/1.1 RFC2616.
The W3C has a page which splits out the Status Code Definitions.
Today, HTTP Status Codes are split across multiple RFCs, with some status codes having their own RFC, e.g. RFC7538 for status code 308, Permanent Redirect.
The Mozilla Developer Network explains
418 I'm a teapot:
418 I'm a teapotclient error response code indicates that the server refuses to brew coffee because it is a teapot. This error is a reference of Hyper Text Coffee Pot Control Protocol which was an April Fools’ joke in 1998.
A useful resource for browsing the codes is the httpstatuses.com site.