Draft says "MAY use HTTP or HTTPS schemes. These URLs MAY have port numbers specified; however, hubs MAY choose to disallow certain ports based on their own policies (e.g., security) and return errors for these requests. The topic URL can otherwise be free-form following the URI spec".
Do you mean MUST, i.e. topic/callback have to be either "http:" or "https:"? I think so. If so, the "MAY" is confusing. I'm not sure the section about port number helps... hubs could apply all sorts of weird policies based on one part or another of the URI. You could maybe shorten this whole section without loss of value by saying something like: "The topic and anchor values MUST be URIs [RFC3986] and MUST use either the "http:" or "https:" URI schemes. They MUST NOT contain anchor fragments." -T
