public function minifyCSS() {
      $css_dir = './css';
      if (is_dir($css_dir)) {
         foreach (glob("$css_dir/*.css") as $css_file) {
            if (!$this->endsWith($css_file, 'min.css')) {
               $this->taskMinify($css_file)
                  ->to(str_replace('.css', '.min.css', $css_file))
                  ->type('css')
                  ->run();
            }
         }
      }
      return $this;
   }