Service Worker
A background script that lets a site intercept network requests, cache responses and work offline.
Service workers sit between the page and the network. They power offline mode, background sync and push notifications, and they're what make progressive web apps feel like installed apps.
From a privacy standpoint, a service worker can outlive the tab that installed it and see subsequent requests to the same origin. Modern browsers unregister them when site data is cleared, which is a good reason to clear data periodically on sites you no longer use.