Accessibility Contrast Optimizer WCAG

A contrast ratio is a luminance comparison, not a color comparison. Each sRGB channel is first linearized - values at or below 0.03928 are divided by 12.92, higher values are raised through ((c + 0.055) / 1.055) to the power 2.4 - and the linear channels are then combined as L = 0.2126R + 0.7152G + 0.0722B. Those coefficients come from the ITU-R BT.709 luminance weights and reflect that the human retina is far more sensitive to green light than to blue. The ratio (L1 + 0.05) / (L2 + 0.05) adds a 0.05 flare term representing ambient light reflecting off the screen, which bounds the scale between 1:1 for identical colors and 21:1 for pure black on pure white. WCAG 2.1 Success Criterion 1.4.3 sets 4.5:1 as the Level AA floor for normal text and 3:1 for large text, defined as 18 point or 14 point bold, which most browsers render at about 24px and 18.66px. Level AAA under SC 1.4.6 raises those to 7:1 and 4.5:1, and SC 1.4.11 requires 3:1 for icons, form field borders and other non-text elements. The main limitation is that the formula is polarity-blind and hue-blind: it scores light-on-dark and dark-on-light pairs identically even though they read differently, it can pass thin or low-weight fonts that remain hard to read, and it says nothing about whether two colors are distinguishable to someone with deuteranopia.

WCAG 1.0, published as a W3C Recommendation in May 1999, addressed color only loosely, advising authors to ensure that foreground and background combinations provide sufficient contrast and offering brightness-difference and color-difference algorithms in supporting techniques rather than a normative threshold. That approach proved hard to test consistently, and the Web Content Accessibility Guidelines Working Group replaced it for WCAG 2.0, which became a W3C Recommendation on 11 December 2008 and introduced the relative luminance ratio used today along with the 4.5:1 and 7:1 numbers. The 4.5:1 figure was not arbitrary. The Understanding document for SC 1.4.3 explains that it was chosen to compensate for the contrast sensitivity loss typical of roughly 20/40 vision, the level commonly associated with advanced age, while 7:1 targets approximately 20/80 vision. WCAG 2.1, published on 5 June 2018, extended the model beyond text with SC 1.4.11 Non-text Contrast at 3:1, and WCAG 2.2 followed on 5 October 2023. The thresholds were then absorbed into regulation: the United States Section 508 refresh adopted WCAG 2.0 Level AA in January 2017, and the European standard EN 301 549 references the same criteria. Work on WCAG 3 has explored the Advanced Perceptual Contrast Algorithm, developed by Andrew Somers, which models font size, weight and polarity rather than luminance alone.