Web Things, Mark Baker points to some original work on XML-RPC from 1998. This reminded me of some earlier work on XML over HTTP that was published on the W3C site. Here's an interesting quote from that paper:
Predicating the system on HTTP, URIs, and XML tightly constrains the solution set thereby increasing interoperability.
Another great quote:
One of the principles of this design is that if a client needs asynchronous notification then this should be accomplished via the HTTP protocol. This implies an HTTP daemon on the client. By unifying the Web client browser with a small (code of less than 2K in size has been realized) HTTP daemon, notification can be realized without undesirable timed polling, a bandwidth wasting technique which is beginning to appear more often.
Ah, those were the good old days... but the irony doesn't end. The current WHAT-WG is actively defining a new protocol for network connections within a browser, in order to provide a stateful asynchronous protocol for bidirectional realtime communication capabilities. I just say, put an HTTP listener inside the browser instead - httpd on the client.