No, there’s nothing wrong in the dev console on the localhost client/browser. HTML is successfully returned from the forum server, I can see it just fine in the dev tools.
Here’s what it should look like when it’s working (some text censored):
Here’s what it looks like instead (some text censored):
So, it’s just that the forum server returns 400 and “Error Embedding” instead of the proper contents.
Sure, but as you suggested, perhaps Discourse is blocking this type of request from referrers that are from localhost. I know “localhost” is not a domain name but a hostname. This is in line with the fact that what you configure for “Allowed Hosts” in the Embedded settings in the forum are just that, hosts.
I know perfectly well what localhost is, that’s not a problem.
Explain what localhost is? No, that’s not needed. When I said that we need to come to a conclusion as to why it’s not working for me, and that it has to be documented, I’m talking about in general why this isn’t expected to work, if that is the case. Again, as it is right now, there’s nothing suggesting that this is expected behavior.