How push services know whether client browser is online or not?

Push notification is implemented through push service. Servers send notification to push services. If the client is offline, the notification is then queued on the push service until client comes online.

My question: how does a push service know whether a client is online or not? Does the client browser notify the push service? If so, does this mean “push notification” is inherently just “pull notification” with a separate queuing service?


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