/**
 * Normalizes CSS by processing nested selectors into flat CSS.
 *
 * Uses PostCSS with postcss-nested to convert nested CSS syntax
 * into standard flat CSS rules.
 *
 * @param cssString - CSS string with nested selectors
 * @returns Promise resolving to normalized CSS string or null on error
 */
export declare function normalizeCSS(cssString: string): Promise<string | null>;