For screen with a pixel ratio higher than 1 (e.g. Retina displays on MacBook and iOS devices) we should provide a higher resolution favicon than we're doing right now.
Instead of 16x16, we need 32x32.
I'm not sure whether a simple 32px PNG (replacing the 16px PNG we are serving right now) works as expected.
We might have to create an .ICO instead, which would contain both.