HTTP 404 - Wikipedia
The HTTP 404, 404 Not Found, 404, 404 Error, Page Not Found, File Not Found, or Server Not Found error message is a Hypertext Transfer Protocol (HTTP) standard response code, in computer network communications, to indicate that the browser was able to communicate with a given server...
404 Not Found - HTTP | MDN
RFC 7231, секция 6.5.4: 404 Not Found. Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content. The compatibility table in this page is generated from structured data.