A 404 page is a recovery interface, and on most sites it is treated as a decoration nobody measures.
Measure it before designing it
Track 404 page views and what happens next. A meaningful volume of visitors hitting errors and leaving immediately is a quantified revenue problem rather than a technical footnote, and that framing is what gets it prioritised.
Fix the sources, not just the page
The 404 page handles the symptom. The report of which URLs are producing errors, and which pages link to them, is where the actual fix is — usually a handful of broken internal links or a migration that missed a section.
Give it real recovery paths
Search, popular destinations, and where possible a suggestion based on the requested URL. A branded page saying "page not found" with a link to the homepage is a dead end with better typography.
Confirm it returns a 404 status
The most common technical error here is a custom error page returning HTTP 200. Search engines then treat every missing URL as a real page with identical content, which manufactures duplication at scale. Check the status code, not just what renders.