Chrome uses Google Cloud Messaging platform (soon to be FCM) to deliver web push notifications to end-users. 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.