1 issue tagged with this topic.
WCAG 2.1's contrast ratio formula was derived from a 1988 luminance model that systematically underestimates low-spatial-frequency text legibility and overestimates legibility for small, thin-weight text. The successor standard — APCA (Advanced Perceptual Contrast Algorithm) — corrects these errors with a model calibrated against psychophysical research. This issue explains what changes, what stays the same, and how to audit a design system under both standards during the transition period.
