Separate the technical response from visible wording
HTTP 404 means that the requested resource was not found. A page headed “Not found” may nevertheless return a different technical response. Use an appropriate network tool to check whether the response matches the situation. MDN explains this status code; its actual configuration depends on hosting or server software.
A file named 404.html is not automatically used for missing addresses in every environment. Establish the required error-page configuration and test it in the real environment. Successfully opening that file directly proves only that the file itself is reachable.
Offer a few suitable routes
State clearly that the requested page could not be found. Do not automatically blame the visitor; an outdated link may be responsible. Offer the homepage and perhaps an important content section. An existing search can help, but must genuinely work if offered.
Avoid listing every destination on the website. Someone needs orientation after an interrupted journey. If you provide contact for reporting the problem, make sure it is reachable and does not itself lead to another missing page. Mentioning the current address that can be copied may help with reporting.
Test nested addresses too
Request a missing address immediately below the homepage and another nonexistent address deeper in the structure. Images, styling and links on the error page need to work in both cases. Relative resource paths may resolve differently according to the requested address, revealing problems only in the deeper version.
Then request an ordinary existing page. An overly broad configuration must not accidentally send valid content to the error page too. Record the addresses and responses checked. The error page is ready when it appears understandably in the actual failure case and its offered routes remain usable.
