What the ratios mean
Contrast ratio ranges from 1:1 (identical colours) to 21:1 (pure black on pure white). WCAG 2.1 sets the bar at 4.5:1 for normal text and 3:1 for large text (18.66px bold, or 24px and up) to pass AA — the level most accessibility laws require. AAA raises it to 7:1 and 4.5:1 respectively.
Why it matters
Roughly one in twelve men and one in two hundred women have some colour-vision deficiency, and far more people read on bright screens outdoors or with ageing eyes. Meeting AA contrast isn't just compliance — it measurably improves readability for everyone.