ResponsiveImage.php

<?php

namespace Drupal\responsive_image\Element;

use Drupal\Core\Render\Element\RenderElement;

/**
 * Provides a responsive image element.
 *
 * @RenderElement("responsive_image")
 */
class ResponsiveImage extends RenderElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        return [
            '#theme' => 'responsive_image',
            '#attached' => [
                'library' => [
                    'core/picturefill',
                ],
            ],
        ];
    }

}