How contrast checker works
WCAG 2.x: linearize sRGB at 0.04045, L = 0.2126R + 0.7152G + 0.0722B. Contrast = (lighter L + 0.05)/(darker L + 0.05). AA normal text requires 4.5:1; large text 3:1.
Try an example
Black on white → 21:1.
Good to know
Pass/fail uses the unrounded ratio. Large means at least 18pt regular or 14pt bold. Opaque solid colours only; no transparency or gradients.
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