• By far the single most-fruitful tactic has been "just look at raw GET responses from Mastodon and see what things are shaped like." I know that "ActivityPub is under-specified" is a bit of a meme, but it's wild how little prior art there is.
• Something that gives distinctly bad vibes: ostatus.org (which ostensibly describes a bunch of the schemae for subscription interactions) now redirects to a gambling site. See more.
• Webfinger is not part of ActivityPub per ce, but it makes sense as an initial beachhead.
• Here's a fun, interesting, vaguely audacious top-level goal: every subscription form on the web should take an ActivityPub username as well as an email address. Ghost has this in their mocks as well, and I don't think it's technically that complex — Hugh Rundle walks through the steps but, if my understanding is correct, he unnecessarily complicates things a little bit. All we really need to do is hit webfinger and then redirect. (Maybe a higher-level question: how do you determine if [email protected] is an email address or an AP username?)
• This screed is a pretty interesting bear case on the scalability of ActivityPub. I am not super worried about the implications here — I get and agree with where the author is coming from, but as an implementer this feels like a "cross this bridge when I get to it" situation.