1xx Informational
Interim responses — the server is still working on it.
4 status codes
100 Continue
Keep going — the server has seen your headers and isn't stopping you.
101 Switching Protocols
The server agreed to change the rules of the conversation.
102 Processing
dep
"Still working on it" — a heartbeat that never became standard.
103 Early Hints
A heads-up before the real response — start preloading now.