Documention
Howo To Fix WordPress 404 Errors (Complete Guide)
Error 404 – what is it and how do I fix it?
How to Create a 404 Error Page in HTML
The importance of a 404 page
404 page examples you need to see before writing your own.
Getting 404 Error on my Published Project - Bugs - Forum
404 Page Not Found Error: What It Is and How to Fix It
Why 404 Error Pages Really Matter – And How You Create Them
The best UX for 404 error page [Practices + Examples]