import { ScaleContext, ScaleUnit } from '../types';
/**
 * Scales a font size depending on a context's width/height
 * @param {number} fontSize size of the font to scale
 * @param {ScaleUnit} [scaleUnit=0.39] the scaling unit (uses "width" as unit if a number is supplied)
 * @param {ScaleContext} [ctx=viewport] the context where to apply the scaling
 * @returns {number} the fontSize scaled
 * @see https://matthewjamestaylor.com/responsive-font-size#fluid
 * @public
 */
export declare function scaleFontSize(fontSize: number, scaleUnit?: ScaleUnit, ctx?: ScaleContext | undefined): number;