I think I may have found a bug --
I've been testing on my Macbook Pro Retina -- when I'm using it by itself, the devicePixelRatio is 2 (retina display). But when I have it plugged-in to an external monitor (Apple Cinema Display) -- that's a non-retina display and the devicePixelRatio is 1.
So if I first view a site on the internal (retina) display, the cookie gets set. But when I view the same site again on my Cinema Displaa (non-retina), the cookie has already been set and the site loads all of the retina graphics.
So I believe the clear_eyes.js needs to be tweked to run a check even if the cookie has already been set, since in rare circumstances, the value may change.
I'll take a stab at coming up with a solution and submit a pull request. Thanks!