/**
 * Adds css to the root node using `adoptedStyleSheets` in modern browsers.
 *
 * Pass an optional key to distinguish multiple stylesheets from each other.
 *
 * Returns a cleanup function to remove the styles.
 */
export declare function loadStyles(rootNode: Document | ShadowRoot, { css, key }: {
    css: string;
    key?: string;
}): {
    loaded: boolean;
    cleanup: () => void;
};