APCA (WCAG 3 draft)

Andrew Somers' SAPC-based contrast algorithm. Returns a signed Lc value on a roughly [-108, +106] scale.

swatch.apcaContrast("#000000", "#ffffff");  //  106.04  (BoW)
swatch.apcaContrast("#ffffff", "#000000");  // -107.88  (WoB)
swatch.apcaContrast("#767676", "#ffffff");  //   71.6   (fails body-text min)

Sign: positive = dark text on a light background; negative = light text on a dark background.

Typical body-text thresholds: |Lc| ≥ 75 is comfortable for sustained reading; ≥ 60 is the practical minimum.

APCA runs alongside the WCAG 2.1 ratio in §04 of the playground.