Measure the experience and prove the offline boundary

First, build a mental model

Offline support is a contract about which required resources are available without a network. Observability is evidence that helps explain actual behavior within a defined environment.

1. localStorage can retain small local state, but it does not cache HTML, JavaScript or media. A manifest describes installation metadata; it does not independently implement an offline application.
2. A service worker can intercept requests and manage caches within its scope. Its installation, activation and update lifecycle must be designed; secure contexts are required, with local development exceptions.
3. Test both a previously visited route and a cold route after disconnecting. A lazily loaded runtime that was never cached can fail offline even when the home page works.
4. Measure load duration, interaction latency, frame time, errors and resource failures with a known device/network scenario. Collect the minimum useful diagnostics and avoid leaking personal data, tokens or private payloads.

export async function measureLoad(load) {
  const started = performance.now();
  try {
    const value = await load();
    return { ok: true, durationMs: performance.now() - started, value };
  } catch {
    return { ok: false, durationMs: performance.now() - started };
  }
}
// Does not log the loaded value or private error details.

1. Scenario

   Name the route, device, connection and cache state before measuring.

2. Observation

   The wrapper records real elapsed time and whether the operation resolved. It does not assume a rejected operation succeeded.

3. Boundary

   A single timing is not a Core Web Vitals assessment, offline guarantee or diagnosis of every cause.