Using a HTML/CSS rendering machine for user interfaces seem popular these days as it gives extended ways for customization using style sheets. And many people are used to hack CSS files from the web which extends the group of possible contributors. Several clients use it for the chat window, Adium on Mac OS X, Gtalk on Windows, Kopete on linux.
To my knowledge, these clients rely on platform specific rendering APIs, and doing this on an x-platform client like Coccinella would be very difficult and create a very large maintenance burden. Since Coccinella is using Tk for its user interface, which is an extremely portable toolkit and has a very high level API, this would require a HTML/CSS rendering widget written entirely for Tk. Fortunately, such a project is running and is called Tkhtml3 (Tcl/Tk projects usually take very straight names). People who know this business understand that writing such a widget from scratch is a giant undertaking, and not many toolkits have succeeded.
Tkhtml3 is still a fast moving target and many things aren't working as they should, but it looks very promising. I couldn't resist adding support for styled chats in Coccinella, just for the fun of it.
The only docs I had was a page from Kopete which I don't know how well it reflects any kind of standard, if there is any. It is clear that the chat style theme directory structure resembles a Mac OS X bundle, and it is not far fetched that it comes from Adium. As I've already said, it is an experiment which looks promising, and not all themes render correctly. But it is x-platform! I built it on Mac OS X (10.2), and on MinGW on Windows just with configure/make. Straight out of the box! Antonio had it running on Linux without any instructions.