/**
 * WCAG 2.x relative luminance of a `#rrggbb` hex colour.
 *
 * Each sRGB channel is linearised (c/12.92 below the 0.03928 knee, otherwise
 * ((c+0.055)/1.055)^2.4) and weighted 0.2126/0.7152/0.0722 for R/G/B.
 *
 * @param hex - colour as `#rrggbb`
 */
export declare function relativeLuminance(hex: string): number;
/**
 * WCAG contrast ratio between two `#rrggbb` colours: (Lmax+0.05)/(Lmin+0.05).
 * Symmetric in argument order; ranges from 1 (identical) to 21 (black/white).
 *
 * @param hexA - first colour as `#rrggbb`
 * @param hexB - second colour as `#rrggbb`
 */
export declare function contrastRatio(hexA: string, hexB: string): number;
/**
 * The greyscale stroke for the 1px seam between two adjacent tabs of the same
 * colour - without it the tabs fuse into one block. The grey must reach the
 * target ratio against EVERY given background; the caller passes whichever
 * shades the seam can actually touch.
 *
 * Scans grey levels toward the extreme and returns the FIRST level that sits
 * on the required side of every background (lower luminance for 'darker',
 * higher for 'brighter') AND passes against all of them: for 'darker' (light
 * themes) the lightest passing dark grey, for 'brighter' (dark themes) the
 * dimmest passing bright grey - the subtlest line that still separates the
 * tabs. Falls back to `#000000` / `#ffffff` when no such level exists.
 *
 * @param backgrounds - `#rrggbb` shades the seam must contrast with
 * @param direction - 'darker' for light themes, 'brighter' for dark themes
 * @param target - required contrast ratio (WCAG non-text minimum is 3:1);
 *   always passed explicitly - the magnitude-to-ratio mapping lives in one
 *   place, CONTRAST_TARGETS in dynamicStyle.ts, and a default here would be
 *   a second, driftable copy of it
 */
export declare function dividerGrey(backgrounds: string[], direction: 'darker' | 'brighter', target: number): string;