of a push message to a webapp via a push service. An application server can send a push message at any time, even when a webapp or user agent is inactive. The push service ensures reliable and efﬁcient delivery to the user agent. Push messages are delivered to a Service Worker that runs in the origin of the webapp, which can use the information in the message to update local state or display a notiﬁcation to the user. This speciﬁcation is designed for use with the web push protocol, which describes how an application server or user agent interacts with a push service.