WebKit

WebKit is the Apple technology that powers the Safari web browser. It also lets applications make use of a sophisticated HTML and CSS rendering engine for beautiful display of content.

Adium uses WebKit to provide a customisable display of messages in the chat window. Message Styles are plug-in bundles which contain HTML, CSS, and JavaScript to tell the WebKit Message View how to render the messages. Hundreds of message styles are available from the  Adium Xtras site.