&gt; Is there any short explanation how this works under the hood?<p>Yes, the spec is the most detailed description of what goes on under the hood and it&#x27;s not too long: <a href="https:&#x2F;&#x2F;www.w3.org&#x2F;TR&#x2F;push-api&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.w3.org&#x2F;TR&#x2F;push-api&#x2F;</a>, but here&#x27;s my attempt at answering each of your questions:<p>&gt; Does the browser maintain an open connection (like a Websocket) for each subscription, or there are some brokers, or if it can use OS facilities (like GCM&#x2F;APN on mobile) when available, etc etc.<p>Yes. Chrome maintains an open connection to Google&#x27;s push servers, Firefox maintains an open connection to Mozilla&#x27;s push servers.<p>&gt; In particular, I&#x27;m curious of privacy implications. When I subscribe - what site knows of me (say, if that&#x27;s a direct connections, I&#x27;m not sure I want to tell every site out there when I&#x27;m online), or if any third parties are involved - what they know and do (say, what&#x27;s about sender and receiver authentication)?<p>The Push connection itself would not reveal whether you are online since it&#x27;s made to Google&#x2F;Mozilla and not other individual sites, however the presence of a Service Worker could be used to determine this. For example, a website could send a background notification and check your network connectivity from the Service Worker without necessarily showing you a visible notification. This is already true for push notifications most people already receive from mobile apps.<p>A common use for these kinds of background notifications would be to do background data loading (such as loading new email messages to view later offline).