async function getStylesheets(files, options, pages) {
    if (options.stylesheets && options.stylesheets.length) {
        /* Simulate the behavior below */
        return [files, options, pages, [options.stylesheets]];
    }

    /* Extract the stylesheets from the HTML */
    const stylesheets = await Promise.all(pages.map(page => jsdom.getStylesheets(page.window, options)));

    return [files, options, pages, stylesheets];
}
Get the contents of CSS files.
@param  {Array}   files   List of HTML files
@param  {Object}  options UnCSS options
@param  {Array}   pages   Pages opened by jsdom
@return {Promise}