michael, 
I don't understand colour contrast calculations:
I got 2 buttons with red background. One with black text, the other with white.
colour contrast ratio with the black on red button is supposedly 4.81, while white on red is only 4.35 (which crucially is below the 4.5 threshold)
Yet, to my eye the white on red is much more legible than black on white.
What am I missing?