Resilience

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.

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.

Core content and primary navigation should keep working when JavaScript fails to load or throws — treat client-side scripts as an enhancement, not the delivery mechanism.

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.

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.

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.

When you retire an endpoint, announce it in machine-readable form: the Deprecation and Sunset response headers tell clients it is going away, when, and where to go next — so integrations migrate before anything breaks.