@nhumrich @ballpointcarrot WebSub is server-server via webhooks yes.
For server-browser, use EventSource^1 & Notifications^2 APIs.
Re: “Most common”, I see more periodic polling from servers (bots, crawlers) than browsers in my own server logs. Your logs may vary.
^1 https://developer.mozilla.org/en-US/docs/Web/API/EventSource
^2 https://developer.mozilla.org/en-US/docs/Web/API/Notifications_API/Using_the_Notifications_API