Sure, you can do that. I think I have the answer though. Both sh.itjust.works and feddit.nu have the issue where they return html when they should return json if the Accept header isn't a very specific string. There's a patch for the nginx config to solve it available but it seems they've not applied it yet.
You can test any instance for this by doing the following.
Find a local user on the instance and get their page (https://instance.tld/u/username).
Find a linux (or any OS that has it) with curl installed and type the following:
curl -X GET -H 'Accept: application/activity+json,application/json' https://userlink
If the result is html they have not patched, if it is json, they have patched.
curl -X GET -H 'Accept: application/activity+json' https://userlink
Will return json all the time on all instances, and I've patched my kbin to use this Accept header until all are fixed.