What if an HTTP/1.1 client talk to an HTTP/2 only server and what if an HTTP/2 client talk to an HTTP/1.1 only server?

Question

HTTP/2 is definitely the future trend because it is now the standard of HTTP protocol. As we can see in Can I use, 70.15 percent of browsers support the HTTP/2. But HTTP/2 i

Answer 1

If a browser only supports HTTP/1.1 and the server only supports HTTP/2, they cannot communicate. The server will not recognize what the client sends (in particular there will be no connection preface, which the server treats - following the specification - as a connection error), and will close the connection.

"A browser that only supports HTTP/2" does not exist; if they support HTTP/2, they also support HTTP/1.1. But let's assume that such browser exist.

In this latter case, the server will see the connection preface and will not recognize the PRI method. What exactly the server does in this case depends on the server. It may return a 400 Bad Request, or perhaps just close the connection, or it may trigger an internal server error.