This posting may miss the spirit of this thread but anyhow:
Have you tried using Conkeror (http://www.conkeror.org/)?
As far as I understand this is based on xulrunner, but it behaves like Emacs.
Because it is based on xulrunner it is a fully featured HTML-browser (Java Script, CSS, cookies, forms etc. support). Normally I have an Emacs instance and Conkeror instance open, and because it behaves like Emacs I sometimes forget that I am interacting with the browser.
Wouldn´t it be much more in the UNIX-kind of spirit to not reinvent the wheel but to use and combine the tools all ready existing? Which means would it not be better to work on a really good interface between Emacs and Conkerer and relieve yourself of the work involving the maintenance of top notch Java Script, CSS interpreters
etc.. For me it would be enough to have excellent access to the html-source, the DOM-tree, Java Script code, CSS files, cookies etc. in an Emacs buffers, and have the page rendered by Conkeror.
Kind regards Stefan