"Ah, just to clarify, WebSub isn’t required, it’s just supported — it’s just one of the things Pushl supports. I hadn’t considered offline support as something that someone might want, so for that use case Gruta is definitely a better choice."
Cool, I understand. I'm still in the process of digesting the vast sea of information that is IndieWeb and praise the incredibly active and imaginative people there.
I have some concerns about the risk of spam once these protocols hit mainstream, though. I know the IndieWeb people have this in mind.