example for rendering form for WV

CustomElementsFormController.php

class CustomElementsFormController extends HtmlFormController {

  /**
   * {@inheritDoc}
   */
  public function getContentResult(Request $request, RouteMatchInterface $route_match) {
    $form = parent::getContentResult($request, $route_match);
    return $this->getCustomElementsContentResult($form);
  }

  /**
   * Prepare the content result wrapped in custom elements.
   *
   * @param array $form
   *   The prepared and processed form render array.
   *
   * @return \Drupal\custom_elements\CustomElement
   *   The lupus-form custom element.
   */
  public function getCustomElementsContentResult(array $form): CustomElement {
    return CustomElement::createFromRenderArray($form);
  }

}

/**
Example route definition:

route.example.form:
  path: '...'
  defaults:
    _controller: '...\CustomElementsFormController::getContentResult'
  requirements:
    _permission: 'access content'
    _format: 'custom_elements'

*/