WCAG 2.1 (Web Content Accessibility Guidelines) requires specific contrast ratios between text and background colors to ensure readability for users with visual impairments. AA compliance (minimum for most contexts): 4.5:1 for normal text, 3:1 for large text (18pt or 14pt bold). AAA compliance (enhanced): 7:1 for normal text, 4.5:1 for large text. A ratio of 1:1 is zero contrast (invisible), while 21:1 is maximum (pure black on white). Our checker instantly calculates the relative luminance of both colors and reports whether the pair meets AA and AAA standards.