Just looking at some tutos in both Chrome and Firefox and it seems like the :host rule demo at http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom-201/#toc-style-host does not work as expected in Firefox. (This is with dom.webcomponents.enabled = true).
I'm mostly concerned about the ability to react to user states. Fwiw the first demo seems to not work in Firefox, while the second demo (related to handling user state, does not seems to work in both Firefox nor Chrome 35 on my laptop)
Created attachment 8579159 [details] [diff] [review] Part 1: Allow psuedo class to be optionally functional.
These patch builds on top of things in bug 1082060, so that bug needs to be sorted out first.