Contrast Accessibility Checker

The WCAG contrast ratio compares two colors by relative luminance, not by hue. Each sRGB channel is first linearized, dividing by 12.92 below 0.04045 and otherwise raising ((c + 0.055) / 1.055) to the power 2.4, then weighted 0.2126 for red, 0.7152 for green and 0.0722 for blue to reflect how much each primary contributes to perceived brightness. The ratio is (L1 + 0.05) divided by (L2 + 0.05), producing values from 1 to 1 for identical colors up to 21 to 1 for black on white. The thresholds that follow are success criterion 1.4.3 at level AA, requiring 4.5 to 1 for normal text and 3 to 1 for large text defined as 18 point or 24 CSS pixels, or 14 point bold; criterion 1.4.6 at level AAA raising those to 7 to 1 and 4.5 to 1. The familiar consequence is that number 767676 is the darkest neutral gray that still clears 4.5 to 1 on white. The formula has documented weaknesses. It is symmetric, so swapping foreground and background returns the same figure even though dark text on a light field is measurably easier to read than the reverse at equal ratio. The Accessible Perceptual Contrast Algorithm reported here as an Lc value addresses those gaps: it is polarity aware, spans roughly minus 108 to plus 106, and its published guidance sets about Lc 75 for small body text, Lc 60 for larger body copy, and Lc 45 for headline sizes. APCA remains draft work, so the AA and AAA ratios stay the compliance target.

The first W3C Web Content Accessibility Guidelines reached Recommendation status in May 1999, but they did not use a ratio at all. Their color check relied on two separate thresholds, a brightness difference of at least 125 and a color difference of at least 500 computed on raw channel values, a formula that produced inconsistent results. WCAG 2.0, published in December 2008, replaced it with the luminance based ratio still in use, grounded in the same photopic weighting that underlies broadcast luma and the CIE Y coordinate. WCAG 2.1 followed in June 2018 and added criterion 1.4.11 for non-text contrast, closing the gap that had left icons, form borders and chart elements unregulated, and WCAG 2.2 arrived in October 2023. Regulation is what turned the numbers into obligations. The United States Access Board issued its Section 508 refresh as a final rule in January 2017, incorporating WCAG 2.0 level AA by reference for federal information technology, and the European standard EN 301 549 does the same for public sector procurement under the EU Web Accessibility Directive of 2016. Meanwhile research within the W3C Silver task force, the group drafting WCAG 3.0, documented combinations that satisfied the 2.x ratio yet tested poorly with real readers, particularly in the middle of the lightness range and in dark mode. Andrew Somers began developing APCA around 2019 in response, building a perceptual model that accounts for polarity and for spatial frequency of the text.