About the contrast checker
WCAG contrast ratios decide whether real users can read your text - and whether your product is legally accessible. The checker computes both the classic WCAG 2.1 ratio (1:1 to 21:1) and the modern APCA Lc score (-108 to +106) for any foreground / background pair. WCAG is required by law in most jurisdictions; APCA is the perceptually-tuned algorithm shipping with WCAG 3 drafts and is increasingly adopted by Apple, Adobe and the larger design systems.
Use the single-pair view to debug a specific token, or open the matrix view to audit every text-on-surface combination in your palette in one screen. Both views surface the WCAG thresholds (3:1 large, 4.5:1 body, 7:1 AAA) and APCA polarity-aware bands (Lc 60+ body, Lc 75+ comfortable, Lc 90+ rich). The matrix is the fastest way to spot the failing pairs in a design system before they ship - most palettes have one or two cells that need a small lightness nudge.
When a pair fails, send the colors to the A11y Fixer, which suggests the smallest perceptual shift that brings the pair to compliance. To learn the underlying math, read WCAG Contrast, Explained Without the Jargon and APCA vs WCAG 2.1 Contrast: Which Should You Use?
Related: A11y Auto-Fix · Colorblind Simulator · Generate Accessible Tokens