This web protocol is becoming more and more popular considering its' value, but it's not in the list of tags.
How should a question be tagged since this tag isn't here?
Since the protocol is used by a browser, I think mentioning the browser would be enough. In the same question it could be mentioned that the OP is trying to do something related to SPDY.
If not, this could extent in a bad way to too many other tags that include WebRTC, HTML5 (File and Audio and Web Storage to name a few), WebGL and anything else that a browser offers (Updated browsers that is) or appears in the future for it.
Depends on the context of the question. A lot of servers (i.e. server software) are going to adopt it (SPDY) eventually. I'd go with
server for now and if you ask about a specific software, such as
nginx, then add that.
On the client side go with Luis' answer.