HTTP/3 is absolutely related to past HTTP protocols: it’s built upon HTTP semantics.
HTTP specs are also structured this way now:
• RFC 9110 defines HTTP semantics;
• RFC 9111 defines HTTP caching behaviour;
• RFC 9112 defines HTTP/1.1;
• RFC 9113 defines HTTP/2;
• RFC 9114 defines HTTP/3.
(The previous edition, RFC 7230–7235 for HTTP and HTTP/1.1, plus RFC 7240 for HTTP/2, was really a bit of a weird structure, a regression for practical use compared to RFC 2616 which 7230–7235 obsoleted. Defining version-neutral semantics and then the 1.1, 2 and 3 wire formats is a very sensible design.)
HTTP specs are also structured this way now:
• RFC 9110 defines HTTP semantics;
• RFC 9111 defines HTTP caching behaviour;
• RFC 9112 defines HTTP/1.1;
• RFC 9113 defines HTTP/2;
• RFC 9114 defines HTTP/3.
(The previous edition, RFC 7230–7235 for HTTP and HTTP/1.1, plus RFC 7240 for HTTP/2, was really a bit of a weird structure, a regression for practical use compared to RFC 2616 which 7230–7235 obsoleted. Defining version-neutral semantics and then the 1.1, 2 and 3 wire formats is a very sensible design.)