How to Create a 404 Error Page in HTML
Using Google Analytics to find your 404s
Make a better use of your 404-page
Why do I see a Page not found (404) error? – start.me Help Centre
404 page U.S. Web Design System (USWDS)
About the Wix 404 Error Message Page, Help Center
A page that Google can't find !. yes that's correct and here is the page with reason…, by Millan, Medium
10 examples of custom 404 pages ranked from best to worst, by Tia Loehnert, CCBA
How to edit the 404 Not Found template