APCA

APCA is the contrast formula that actually reflects how human vision reads text. WCAG 2 contrast ratios use a 30-year-old luminance-based equation that ignores font weight, size, color temperature, and polarity (light text on dark backgrounds behaves differently than the reverse). APCA was built to fix those gaps and is currently proposed as the contrast standard in the WCAG 3 draft. APCA scores range from 0 (no contrast) to roughly 108 (extreme contrast). Unlike WCAG's single-ratio thresholds, APCA uses ranges tied to use case. Body copy at 16px regular needs 75+. Small body at 14px needs 90+. Large text at 24px+ can pass at 60+. Non-text UI needs at least 45. Heavier font weights lower the required score. Lighter fonts raise it. APCA is not yet legally required. WCAG 2 AA remains the compliance floor in every jurisdiction that references WCAG. But shipping products, especially those with design-conscious audiences, use APCA internally because it correlates better with what actually reads well. Ignoring APCA is fine. Using it as a second filter on top of WCAG is how modern systems (Radix, Material, Spectrum) avoid the false-pass problem where a pair hits 4.5:1 and still looks painful. For working designers: keep the WCAG checker for compliance reports. Keep the APCA calculator open for every new token pairing. Ship what passes both.