There is a way to run your own code inside the Linux kernel — attached to a syscall, a network packet, a function entry — without writing a kernel module and without rebooting. That sounds like it should be impossible or insane, and it's neither. It's eBPF, and it's quietly become the foundation under nearly every observability, networking, and security tool you already use.
HTTP/3 didn't invent new headers or smarter routing — it kept HTTP exactly the same and swapped the pipe underneath. Instead of TCP plus TLS, it runs on QUIC over UDP, and that one change fixes head-of-line blocking, cuts the handshake to a single round trip, and lets a connection survive you walking from Wi-Fi to cellular.