When background_sync is enabled, the SW intercepts POST form submissions. If the network is unavailable, the request is serialised into IndexedDB (opwa-sync-db, store offline-forms). On reconnect, the SW's sync event replays each queued request to the original form action URL.
This is useful for contact forms, newsletter signups, and other POST endpoints that should not be lost offline.
Browser support: Background Sync is supported in Chrome/Edge. Safari does not yet support the Background Sync API. The SW falls back to a network-online listener in unsupported browsers.