Question 1

What is the difference between 401 and 403?

Accepted Answer

401 Unauthorized means you are not authenticated (no or bad credentials). 403 Forbidden means you are authenticated but not allowed. The decoder flags this gotcha.

Question 2

Is this the full IANA registry?

Accepted Answer

No — it is a curated set of the codes you actually meet in practice, with practical guidance. For the exhaustive registry, see IANA.