You can't get 0-RTT with TCP because the TCP handshake itself isn't 0-RTT. HTTP/3 accomplishes it by switching to UDP -- but then you can't get through the lega

You can't get 0-RTT with TCP because the TCP handshake itself isn't 0-RTT. HTTP/3 accomplishes it by switching to UDP -- but then you can't get through the legacy middleboxes that only allow HTTPS over TCP port 443.