code: public function addClass($value) : self
      {
          $currentClasses = $this->getClasses();
          $classes = array_unique(array_merge($currentClasses, (is_array($value) ? $value : explode(' ', $value))));
  
          return $this->addAttribute('class', implode(' ', $classes));
      }
docs: @param array|string $value
  
  @return $this|self