1xx - this is the websocket connection if you want to watch what it's doing

418 I'm a teapot seemed pretty novel the first few times.

1xx is the response code you try to ignore because it's really annoying to deal with properly (like when you receive a 100 response to a file upload, or even worse, a 101 with a protocol that isn't exactly what you expected, or a 103 which means more content comes later).

1xx response codes are crucial for things like WebSockets and HTTP/3 but they make the HTTP state machine more complex and many people writing manual HTTP requests are ill-prepared to deal with them.