template_preprocess_fieldset

template_preprocess_fieldset(&$variables)

function template_preprocess_fieldset(&$variables) {
  $element = $variables['element'];
  Element::setAttributes($element, array('id'));
  RenderElement::setAttributes($element);
  $variables['attributes'] = isset($element['#attributes']) ? $element['#attributes'] : array();
  $variables['prefix'] = isset($element['#field_prefix']) ? $element['#field_prefix'] : NULL;
  $variables['suffix'] = isset($element['#field_suffix']) ? $element['#field_suffix'] : NULL;
  $variables['title_display'] = isset($element['#title_display']) ? $element['#title_display'] : NULL;
  $variables['children'] = $element['#children'];
  $variables['required'] = !empty($element['#required']) ? $element['#required'] : NULL;

  if (isset($element['#title']) && $element['#title'] !== '') {
    $variables['legend']['title'] = ['#markup' => $element['#title']];
  }

  $variables['legend']['attributes'] = new Attribute();
  // Add 'visually-hidden' class to legend span.
  if ($variables['title_display'] == 'invisible') {
    $variables['legend_span']['attributes'] = new Attribute(array('class' => 'visually-hidden'));
  }
  else {
    $variables['legend_span']['attributes'] = new Attribute();
  }

  if (!empty($element['#description'])) {
    $description_id = $element['#attributes']['id'] . '--description';
    $description_attributes['id'] = $description_id;
    $variables['description']['attributes'] = new Attribute($description_attributes);
    $variables['description']['content'] = $element['#description'];

    // Add the description's id to the fieldset aria attributes.
    $variables['attributes']['aria-describedby'] = $description_id;
  }

  // Suppress error messages.
  $variables['errors'] = NULL;
}