The core specifications for XMPP are developed at the Internet Engineering Task Force (IETF) - see RFC 6120, RFC 6121, and RFC 7622 (along with the WebSocket binding defined in RFC 7395).
The XMPP Standards Foundation develops extensions to XMPP in its XEP series. This page lists Draft and Final XEPs as well as experimental proposals that are currently under consideration. A list of all XEPs (including retracted, rejected, deprecated, and obsolete XEPs) is also available. Good places for developers to start are the client compliance and server compliance definitions, as well as the technology overview pages.