/**
 * Set styles to a HTMLElement.
 * @param dest HTMLElements style property.
 * @param source The styles as JS object.
 * @param importantProperties Name of important style properties. This properties will be set to `!important`.
 */
export declare function extendStyles(dest: CSSStyleDeclaration, source: Record<string, string>, importantProperties?: Set<string>): CSSStyleDeclaration;