Section 6.1: "will" implies no failure mode (and failure mode is not described in 6.1.2)

[trwnh]

The Link header can also be used to identify the author of an HTML page. The author link type identifies the author of a resource. So, a Link header with a rel property equal to "author" and a type property equal to "application/activity+json" will link to the ActivityPub representation of the author of the object.

type="application/activity+json" does not always imply ActivityPub, so we can't always say it "will link to the ActivityPub representation".

More generally, there is a (possibly intentional?) conflation throughout the report between "ActivityPub object" and "ActivityStreams 2.0 object".

At the very least, we might rephrase:

[...] can link to [...]

Or:

[...] may link to [...]

[pfefferle]

Do we have to mention that Mastodon also requires the correct Accept header for the rel-alternate links? mastodon/mastodon#22306

[trwnh]

@pfefferle that seems to be a slightly different issue. this currently filed issue is about it being possible for links to return 404 or to return AS2 that is not AP. but i agree that rel=alternate does not imply that conneg is necessary

[nightpool]

I think per spec retrieving any ActivityPub resource requires conneg. I don't see why we would have to spell that out for this path of discovering an ActivityPub url specifically

[trwnh]

@nightpool AP currently requires "clients" MUST use the Accept header set to application/ld+json; profile="https://www.w3.org/ns/activitystreams" but:

1. not every HTTP agent is an AP client
2. not every HTTP resource with Content-Type: application/activity+json is being served by an AP server
3. AP servers are still free to not require conneg; e.g. they might be configured to respond with the AS2 document always or by default

in the case of encountering a link with rel=alternate type=application/activity+json we might say that the link target has a high chance of being served by an AP server, but we can't say for sure that an HTTP agent MUST add an Accept header equal to the link type during their request. i can see it making sense in some cases but i'm not sure it's something that should always be done as a general rule. it probably makes sense for this report insofar as the reader is interested in implementing what AP Section 3.2 means by "client", at least.