Except it doesn't couple an application and a resource. Serving the application/web UI is only a side effect of the browser requesting the resource be served in HTML format, which brings up the web application. It's a backwards compatible convenience for users to be able to access the resource in more than one way.

If you request the same thing in application/activity+json format then you get exactly the same resource, but in machine format. Lemmy apps use that for example to display the content natively and completely bypass the web application part of it.

The browser very well could say it accepts both ActivityPub and HTML format, in which case the server would serve the ActivityPub that the browser could display using some native implementation. And bam, there's no longer an application+resource tie.

I don’t get what is the “application” in this context. Is that the Lemmy server or kbin server, which use the ActivityPub protocol? And couldn’t this be solved with a sort of .apub at the end of each resource, like the .json used to work for Reddit?