I just had that problem when you browse to a Mastodon post and ⭐️ it, or try to follow someone. The choreography is clumsy, and the kind of thing that will hinder mainstream adoption of ActivityPub.

acct is IANA official and used behind the scenes with webfinger. It'd be dead-simple to enable browsers looking up an app to handle acct: URLs: an ActivityPub client.

It's trickier to think of how to handle posts, given the discussion about Lemmy/Mastodon interop… and the ActivityStreams spec has a dozen object types! But I think I'm going to want only as many clients as necessary, and one sounds great, so I'm interested to hear what people are thinking at an infrastructure level