@jenniferplusplus @apps personally I belive it's fine to assume https as the transport. While ActivityPub may or may not work over telnet I don't think we'd be using a web+ scheme for that.
However - assuming that we can agree on https - my question was just: is it
a) web+activitypub:something
b) web+activitypub://something
The examples in the FEP indicate a although there isn't any normative text.
The web+soup example here https://html.spec.whatwg.org/#custom-handlers also uses variant (a)