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.
For a decade, real-time on the web meant one thing: a WebSocket. But a WebSocket is a single TCP connection, which means one lost packet can freeze every message behind it, and it only knows how to deliver data one way — reliably and in order, whether you wanted that or not. WebTransport is the newer browser API that runs over HTTP/3 and fixes both limits: many independent streams, plus a lossy express lane for data that's worthless once it's stale.