Push notifications
Fastest way to test it is LetReach and it’s free for up to 2000 subscribers. It is from their domain, so you are locked-down to them.
Not working on Firefox, and that is now possible: Web Push Arrives in Firefox
Minishlink/web-push: Web Push library for PHP
WordPress › push « Tags « WordPress Plugins
WordPress › Chrome Push Notifications « WordPress Plugins Dexie.js - Minimalistic IndexedDB Wrapper
Important:
WordPress - only load plugins on pages that need them - position: relative;