Resilience

Graceful failure — error pages, offline, redirects.

• Required Custom error pages (404, 500) — Custom error pages must return the correct HTTP status code, explain what went wrong in plain language, and offer the user a way forward without leaking implementation details.
• Recommended Maintenance pages and 503 — When the site is intentionally offline, return HTTP 503 with a Retry-After header and a page that tells users what is happening and when to come back.
• Optional Offline support and service workers — A service worker can serve a cached offline fallback page when the network fails, keeping the site usable on flaky connections and turning hard failures into graceful ones.
• Recommended Web app manifest — A web app manifest is a small JSON file that tells browsers how the site should appear when installed — its name, icons, start URL, theme colour, and display mode.
• Recommended Monitoring and uptime — Monitor the site from outside your own infrastructure, combine synthetic checks with real user data, and run a status page on a separate host so it stays up when the site does not.