I don’t understand the “correct scaling” part. The outer ring of the square has a mix of gray and darker pixels, it cannot possibly be as bright on average as the center square which is all grey pixels?
It looks like it is supposed to be a centre of 50% grey and a ring of alternating white and black pixels. As the note says: if you're seeing a darker ring (alternating black and grey pixels) your browser is messing up.
