If you've ever run an accessibility audit and seen "contrast ratio fails WCAG AA" — this post explains what that actually means, how the math works, and when each threshold applies. What is color contrast ratio? Contrast ratio is a number between 1:1 and 21:1 that describes how different two colors appear to a human eye. 1:1 = identical colors (white on white, invisible) 21:1 = maximum contrast (black on white) The ratio is calculated from the relative luminance of both colors — a measure of how bright a color appears to human perception, weighted for how our eyes respond to different wavelengths (we're more sensitive to green than blue, for example).…