Chrome and other leading browsers ( including UC Web, Chromium based browsers such Samsung Internet, Opera) use Firebase Cloud Messaging (FCM) to deliver web push notifications to users devices. While push notifications can be delivered even when the user is not on the website, there are other parameters that can directly impact notification delivery. It is also important to understand the difference between Notification delivery and view-ability. The standard responses from FCM ( or GCM ) only tell you about notification messages being "sent" to FCM( Firebase Cloud Messaging). The actual notification delivery can be tracked only by service worker (notifications on the device are shown by service-worker). This can be tracked for all browsers with the exception of Safari. With this article we wanted to understand notification delivery and how that gets impacted over time. Here is quick take at understanding the web push notification delivery process and the various response codes. To push a notification, you need to call GCM API to invoke push notification functionality for a subscriber. GCM further returns response for every subscriber stating if push notification delivery was successful or if there were failures in delivery. There can be multiple reasons for failures however error codes returned by GCM explain exact error conditions in each scenario.