/**
 * WCAG 2.2 contrast maths, kept dependency-free and side-effect-free so it can be exercised from a
 * plain node-environment vitest run (the `unit` project in vite.config.ts) with no DOM.
 *
 * Only the colour notations the theme CSS actually uses are supported — `#rgb` / `#rgba` /
 * `#rrggbb` / `#rrggbbaa`, `rgb()` / `rgba()` (comma or space separated), and the `white` / `black`
 * / `transparent` keywords. Anything else returns `null` rather than guessing, so an unparsed value
 * surfaces as a loud failure instead of a silently-skipped assertion.
 *
 * @see https://www.w3.org/TR/WCAG22/#dfn-relative-luminance
 * @see https://www.w3.org/TR/WCAG22/#dfn-contrast-ratio
 */
export interface Rgba {
    readonly r: number;
    readonly g: number;
    readonly b: number;
    /** 0–1. */
    readonly a: number;
}
export declare const parseCssColor: (value: string) => Rgba | null;
/** Source-over compositing. `backdrop` must be opaque; the result always is. */
export declare const compositeOver: (source: Rgba, backdrop: Rgba) => Rgba;
export declare const relativeLuminance: ({ r, g, b }: Rgba) => number;
/**
 * Contrast ratio of two opaque colours, truncated to two decimals.
 *
 * Truncated rather than rounded, for two reasons. It is conservative at the threshold — rounding
 * would report a true 4.4951:1 as "4.5" and let a failing pair through — and it is what axe-core
 * reports, so a ratio quoted in docs/a11y-contrast.md is digit-for-digit the same number as the
 * one in the axe baseline in docs/a11y-allowlist.md.
 */
export declare const contrastRatio: (first: Rgba, second: Rgba) => number;