WCAG 2.1

The Web Content Accessibility Guidelines 2.1 contrast algorithm produces a single ratio between two colors based on their relative luminance. It is the default contrast metric used by enforce:contrast.

Relative luminance

Each color's relative luminance Y is the dot product of its linear sRGB channels with the Rec. 709 luminance coefficients:

Y = 0.2126 · R_linear + 0.7152 · G_linear + 0.0722 · B_linear

The coefficients reflect the photopic response of the human eye to the sRGB primaries: green dominates, red contributes about a third as much, blue contributes least.

The linear values are produced by the standard sRGB inverse transfer function:

v_linear = v / 12.92                          if v ≤ 0.04045
         = ((v + 0.055) / 1.055) ^ 2.4        otherwise

Both Luminance (packages/core/src/math/Luminance.ts) and ContrastWcag21 (packages/core/src/math/ContrastWcag21.ts) inline this decode rather than calling srgbToLinear, since the call frequency is high enough that the indirection is measurable.

Contrast ratio

Given two relative luminances Y₁ and Y₂:

ratio = (max(Y₁, Y₂) + 0.05) / (min(Y₁, Y₂) + 0.05)

The + 0.05 flare term models ambient screen reflectance and prevents the ratio from going to infinity when one of the colors is pure black. The result lies in [1, 21], 1:1 for two identical colors, 21:1 for pure black on pure white.

The ratio is symmetric. WCAG 2.1 does not distinguish text from background.

Thresholds

Surface	Ratio	Level
Body text under 18 pt (or under 14 pt bold)	4.5 : 1	AA
Large text at or above 18 pt (or 14 pt bold)	3.0 : 1	AA
Body text	7.0 : 1	AAA
Large text	4.5 : 1	AAA
Non-text UI components, focus indicators	3.0 : 1	AA

Logos, decorative imagery, and disabled states are exempt from the contrast requirements.

Math primitive

contrastWcag21 is exported as a singleton from @studnicky/iridis (packages/core/src/math/ContrastWcag21.ts) and called directly:

import { contrastWcag21 } from '@studnicky/iridis';

const ratio = contrastWcag21.apply(foreground, background);

Both arguments are ColorRecord instances. The primitive reads the rgb field of each (sRGB-safe, gamut-mapped if the source OKLCH lay outside sRGB) and returns a number on [1, 21].

Where it appears

• enforce:contrast (packages/core/src/tasks/enforce/EnforceContrast.ts) walks input.roles.contrastPairs and input.contrast.extra. For each pair whose algorithm is 'wcag21' (the default) it calls contrastWcag21.apply(fg, bg) and lifts the foreground's L coordinate in OKLCH until the configured minRatio is met.
• enforce:wcagAA and enforce:wcagAAA in the contrast plugin (packages/contrast/src/tasks/) are convenience wrappers that apply the AA and AAA threshold tables to declared pairs without each pair having to repeat the ratio.
• enforce:cvdSimulate re-evaluates the same contrastWcag21 math against simulated protanopia, deuteranopia, and tritanopia versions of each pair and records warnings when the simulated ratio drops more than 1.0 below the original.