Should I use HTTP 4xx to indicate HTML form errors?

Question

I just spent 20 minutes debugging some (django) unit tests. I was testing a view POST, and I was expecting a 302 return code, after which I asserted a bunch database entitie

Answer 1

You are right that 200 is wrong if the outcome is not success.

I'd also argue that a success-with-redirect-to-result-page should be 303, not 302.

4xx is correct for client error. 422 seems right to me. In any case, don't invent new 4xx codes without registering them through IANA.