Where does this come from, and can I check it?

The 400 series indicates client error status codes within the Hypertext Transfer Protocol standard. The figure is taken from IETF RFC 7231, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.

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Mondo Knows Facts

HTTP status code 404 designates a page that cannot be found on a server

The 400 series indicates client error status codes within the Hypertext Transfer Protocol standard

The HTTP specification developed by Tim Berners-Lee and Roy Fielding uses three-digit numbers to report server statuses. The first digit 4 indicates a client error response, while 04 specifies that the requested URL resource could not be located by the web server. Urban legends claim it was named after room 404 at CERN, but the code was simply part of the HTTP numeric standard.

HTTP 451 example photograph — Yejianfei · CC BY-SA 4.0 · licence · source

Where this comes from

IETF RFC 7231

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Tags: http, web, standards

HTTP status code 404 designates a page that cannot be found on a server: the short answers

Where does this come from, and can I check it?: The 400 series indicates client error status codes within the Hypertext Transfer Protocol standard. The figure is taken from IETF RFC 7231, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.
Is this really a technology fact?: Yes. It sits on the Technology shelf, one of 1,090 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.
What else can I read about it?: The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.