404 page appears when a user hits a non-existent URL. A bland default 404 drives users away. A thoughtful 404 keeps them on site.
Essentials
Clear message, site navigation, search box, popular page links, "Back to home" button.
Design
Illustration, humor, brand voice โ without clutter.
SEO
HTTP status must actually be 404 (not 200 โ that's a soft 404). Apache: ErrorDocument 404. Nginx: error_page 404.
Analytics
Add GA tracking to 404 โ discover which URLs are broken. Redirect the top offenders with 301.
Mobile
Big tap targets, no horizontal scroll.