Would it be considered mean or uncool to force users to update their browsers?

Well I’m not thinking force exactly, but here’s my idea. It’s possible to look to see if a person is using a legacy browser [via Html].If they are using a legacy browser, you can use a different css file. This is almost common practice. Would it be uncool/mean to make that css file incredibly bland, like making everything greyscale? The site would still work on really old browsers it would just not be very pretty. The css would also unhide a message telling them why the site is in greyscale and what they should do to upgrade…. Good idea or just mean?