Early version of WebSockets exposed bugs in HTTP proxies, some of which were security problems: http://www.adambarth.com/papers/2011/huang-chen-barth-rescor... To fix these kinds of problems, the final version of WebSockets does not use a straightforward upgrade but instead has a kludgey handshake and content masking: https://en.wikipedia.org/wiki/WebSocket#Protocol_handshake https://trac.ietf.org/trac/hybi/wiki/FAQ

HTTP/2 doesn't have the same problems because it requires TLS+ALPN, but IIRC that "clean" solution was only arrived at after years of discussion and experimentation.