HTTP/3 only requires a single round trip to establish a secure and reliable connection. Instead of using the TCP protocol it uses UDP and QUIC. However, not all servers support HTTP/3, so the browser still needs to start with a TCP connection and an older version of the HTTP protocol. The server then returns an HTTP header telling the browser that it supports HTTP/3 and the browser switches to that protocol. If another connection is created in the future HTTP/3 can be used from the start.