if(!function_exists('rduSQ7_add_allowed_tags')) {
    function thmlv_add_allowed_tags($tags) {
        $tags['data-reveal-id'] = array(
            'login_panel' => true,
            'register_panel' => true
        );
        $tags['class'] = array(
            'text-center' => true,
        );
        return $tags;
    }
    add_filter('wp_kses_allowed_html', 'rduSQ7_add_allowed_tags');
}