function add_rel_preload($html, $handle, $href, $media) {
    if (is_admin())
        return $html;

    $html = <<<EOT
<link rel='preload' as='style' onload="this.onload=null;this.rel='stylesheet'" 
id='$handle' href='$href' type='text/css' media='all' />
EOT;
    return $html;
}

add_filter( 'style_loader_tag', 'add_rel_preload', 10, 4 );   