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 (_javascript_, 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 _javascript_, CSS interpreters
etc.. For me it would be enough to have excellent access to the html-source, the DOM-tree, _javascript_ code, CSS files, cookies etc. in an Emacs buffers, and have the page rendered by Conkeror.
Kind regards Stefan