Web Notifications have an in-page event delivery model by default, which means that notifications cannot reliably outlive the page's lifetime. Allowing event delivery to be routed to a page's Service Worker provides a solid solution.
Docs: http://updates.html5rocks.com/2015/03/push-notificatons-on-the-open-web
Samples: https://github.com/GoogleChrome/samples/tree/gh-pages/push-messaging-and-notifications