Last round we checked this out via the # of logins from each sort of browser. We found it was basically ff2 > IE 6 > IE 7 >> ff 1.5
It was about 101k/99k/83k/39k
This was done on logins not users, and wasn't 100% precise, but it gives some idea.
And though I don't know how many will admit it, there's a fair amount of pirated win XP copies out there that will have problems installing IE 7, hence lack of upgrading to that (though many would use FF2 instead I think).
In terms of what % of the game is CSS, the answer is now a LOT of it. Cin is gradually recoding the pages from tables to divs where appropriate (i.e. most of the time) and all text, menus, headers, and most messages (and even the progress bars..) are all positioned by css.
Examples:
http://game.planetarion.com/images/t...lanetarion.css
http://game.planetarion.com/images/t...lanetarion.css
http://game.planetarion.com/images/t...lanetarion.css
http://game.planetarion.com/images/t...lanetarion.css
http://game.planetarion.com/images/t...lanetarion.css
http://game.planetarion.com/images/t...lanetarion.css
http://game.planetarion.com/images/t...lanetarion.css
In #45 and #44, the metal / crystal / eonium /score/ value / etc, as well as the planet name and ruler name, have all been moved to very non standard positions.