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?
<p>HTTP/2 is definitely the future trend because it is now the standard of HTTP protocol. As we can see in <a href="http://caniuse.com/#search=HTTP2" rel="noreferrer">Can I use</a>, 70.15 percent of browsers support the HTTP/2. But HTTP/2 is so new that there are browsers that only support HTTP/1.x and there are many servers that only support HTTP/1.x. I knew that a client can use HTTP upgrade mechanism to negotiate a proper protocol to communicate with the server. For example, if the server supports HTTP/2, their communicating protocol will switch to HTTP/2, otherwise, HTTP/1.x is used. But this only applies to the situdation where the browser the clients used supports both HTTP/2 and HTTP/1.x, right?</p>

<p>But what if a user on a browser that only supports HTTP/1.x wants to communicate with HTTP/2 only server? Will the server ignore the request or send an error back to the user?</p>

<p>And what if a user on a brower that only supports HTTP/2 wants to communicate with HTTP/1.1 only server? I am thinking the process might go like this: The user sends a connecion preface to the server, the server cannot recognize the request, so the user might receive a connection error message. Is this right?</p>

<p>Or is there any browser that supports only HTTP/2?</p>