public function addClass(?string $class): void
  {
    // If class is empty return immediately.
    if ($class===null || $class==='') return;

    if (isset($this->attributes['class']))
    {
      $this->attributes['class'] .= ' ';
      $this->attributes['class'] .= $class;
    }
    else
    {
      $this->attributes['class'] = $class;
    }
  }