In college I had a networks class where the capstone project was writing a basic HTTP server in C. It's actually shockingly easy, especially if you're only supporting get, and fetch.

Mine was something like 70 lines, and would just listen on 8080 and fork when it got a connection before checking for the requested file and sending it or a 404. I was immediately tempted to try adding something like CGI support but didn't have the time that semester.