WCAG 2.x contrast (4.5:1 for body, 3:1 for large/UI) is the current legal standard but has known issues — it under-flags some pairs and over-flags others. APCA (Accessible Perceptual Contrast Algorithm) is the experimental WCAG 3 candidate that measures perceptual lightness contrast on a -108 to +108 scale. AI2 Design's contrast matrix reports WCAG-style ratios because they're auditable today, but the brief's `accessibilityRules` notes APCA where the captured site uses it.
Glossary · accessibility
WCAG vs APCA
Two competing color contrast standards — WCAG (4.5:1 ratio formula) and APCA (perceptually weighted lightness contrast).
External reference
APCA Contrast