Implying that the individual's browser is not modern might sound offensive. Lots of people aren't allowed to control which browser or which version they use at work.

I would rephrase it as a feature thing to make it more objective. "You appear to be running IE6 which doesn't support X, so you'll miss out on feature Y of this site. To experience Y, please either upgrade your browser or try Chrome/Firefox or Safari" might be a bit more useful to the user.