Fieldgroup.php

<?php

namespace Drupal\Core\Render\Element;

use Drupal\Core\Render\Attribute\RenderElement;

/**
 * Provides a render element for a group of form elements.
 *
 * In default rendering, the only difference between a 'fieldgroup' and a
 * 'fieldset' is the CSS class applied to the containing HTML element. Normally
 * use a fieldset.
 *
 * @see \Drupal\Core\Render\Element\Fieldset for documentation and usage.
 *
 * @see \Drupal\Core\Render\Element\Fieldset
 * @see \Drupal\Core\Render\Element\Details
 */
class Fieldgroup extends Fieldset {
    public function getInfo() {
        return [
            '#attributes' => [
                'class' => [
                    'fieldgroup',
                ],
            ],
        ] + parent::getInfo();
    }

}