Section 3.1: "Given the URL for an HTML document" actually describes a mixed resource

[trwnh]

Given the URL for an HTML document, such as https://mixed.example/some/path/to/note-1, a consumer could attempt to retrieve the corresponding ActivityPub JSON-LD object using this HTTP request

The URL is not strictly "for an HTML document"; it is for a resource which can have varying representations, some of which may be HTML.

I think this should be reworded to be clear about the user story. Has the user agent already fetched the resource and received an HTML response, or is this just some URL encountered somewhere without having requested with a GET/HEAD yet?

I think a better wording would be:

Given the URL https://mixed.example/some/path/to/note-1 which has a default representation as an HTML document, a consumer could attempt to retrieve an ActivityPub JSON-LD object using this HTTP request

Or perhaps even simply:

Given the URL https://mixed.example/some/path/to/note-1, a consumer could attempt to retrieve an ActivityPub JSON-LD object using this HTTP request

[TallTed]

I like the latter better. I would add a comma , following URL.