Because while 100% of currently deployed browsers won't support it when it's available, nearly 100% of the users of those browsers (sans IE6) will upgrade very

Because while 100% of currently deployed browsers won't support it when it's available, nearly 100% of the users of those browsers (sans IE6) will upgrade very soon after a new version is out. IE6 users (especially corp users) will probably not upgrade any time soon. So, while modern browsers might support it, a certain (largish) population will not. It's the NS4 problem. Except worse.