colour
Contrast ratio
WCAG's measure of the luminance difference between two colours – 4.5:1 is the legal-strength minimum for normal text against its background.
A mathematically defined score from 1:1 (identical) to 21:1 (black on white). WCAG 2.2 requires 4.5:1 for normal text and 3:1 for large text and UI components like icons and input borders (7:1 and 4.5:1 for the stricter AAA tier).
The thresholds aren’t arbitrary: 4.5:1 was chosen to compensate for the vision loss typical of roughly 20/40 acuity – about what an 80-year-old experiences – plus common colour-vision deficiencies. These are the only contrast numbers with legal force (ADA litigation, the EU Accessibility Act), which matters when someone proposes “the newer method” – see APCA.