Acid2 was designed before high-density screens were common; it has known bugs when one CSS px is not exactly one device pixel:

https://bugzilla.mozilla.org/show_bug.cgi?id=580920

Actually, on my Mavericks box, Safari 7.0.2 seems to pass Acid2 just fine; the nose even changes color on hover. However, Chrome 35.0.1912.2 pretty obviously fails with the top of the head being messed up and a red line over the forhead. The nose changes color, so that's good. :)

To quote: "Servo passes Acid2 in my branch". It is probably not in default branch yet.