For basic information about HTML, see Wikipedia:HTML.
When you see Kol_Panic talking about serving HTML over PSYC, he's talking about HTML 5, even though psyced already transmits HTML-rendered profiles via PSYC to web-based clients such as PsycZilla. But that's just the beginning of the future of web-based (or rather.. remote GUI?) technology. Several PSYC clients prefer HTML-based GUIs over traditional user interface technologies.
HTML5 is the result of efforts by the Web Hypertext Application Technology Working Group to provide a compatible successor to HTML 4.0 that would more adequately address Web applications. HTML5 is the successor to HTML4, XHTML1, and DOM Level 2 HTML. Just as people often mean mean (X)HTML/DOM/CSS/ECMAScript when they talk about HTML today, the worthwhile thing about HTML5 is that that provides essential hooks for that suite of technologies to provide a two-way interaction with the server.
Understand that HTML5 is not going to remove the limitations of HTTP on its own. It may, however, open the door for web applications to communicate over other protocols, including PSYC, with the server and it will make the case for revisions to HTTP to support server-initiated transfers over the persistent connection established in HTTP 1.1. Even so, HTTP will not offer HTML5 the advantages PSYC does in terms of open federation, multicast routing, shared presence and true push.
The current HTTP-compatible protocol proposal is websockets. This version of the websockets protocol specifies UTF-8 text and uses magic bytes for framing. The initial handshake is an HTTP 1.1 Upgrade request. If the websockets draft becomes a standard, which is likely, it will be a very implementable one compared to others in the psyced portfolio.
Support and Deployment
This assumes one or more PSYC clients, like Dyskinesia, will continue to be built around a GUI that supports Webkit, Mozilla or another HTML5 rendering engine. In the absence of native client support, PSYC may take a similar role with HTTP that it currently takes with IRC and XMPP, that of an enabling technology running invisibly behind the scenes, and PSYC servers may present an even stronger platform for server-based applications as they already are today, in a similar way as it happened for the web.