colorjs.io/src/contrast/WCAG21.js

Let’s get serious about color

// WCAG 2.0 contrast https://www.w3.org/TR/WCAG20-TECHS/G18.html
// Simple contrast, with fixed 5% viewing flare contribution
// Symmetric, does not matter which is foreground and which is background

import getColor from "../getColor.js";
import { getLuminance } from "../luminance.js";

/**
 * @param {import("../types.js").ColorTypes} color1
 * @param {import("../types.js").ColorTypes} color2
 * @returns {number}
 */
export default function contrastWCAG21 (color1, color2) {
	color1 = getColor(color1);
	color2 = getColor(color2);

	let Y1 = Math.max(getLuminance(color1), 0);
	let Y2 = Math.max(getLuminance(color2), 0);

	if (Y2 > Y1) {
		[Y1, Y2] = [Y2, Y1];
	}

	return (Y1 + 0.05) / (Y2 + 0.05);
}