The HTTP specification says that status codes should be three digits integers, but what happens if they are not? April King, head of website security at Mozilla, did some fun experiments to find out.

While it is easy to create test cases for conditions that don't satisfy this requirement, it is somewhat more difficult to determine how third-party libraries will handle HTTP requests that fall outside this constraint. I looked around the internet for websites to help me test weird status codes, but most of them only let me test with the known status codes. As such, I decided to add arbitrary HTTP status codes to my naughty httpbin fork, called misbehaving.site. What I discovered is that the various browser manufacturers have wildly different behavior with how they handle unknown HTTP status codes.

https://pokeinthe.io/2017/09/14/http-status-code-handling/