diff --git a/core/lib/Drupal/Core/Render/Element/Optgroup.php b/core/lib/Drupal/Core/Render/Element/Optgroup.php
index 03bc474..7e43234 100644
--- a/core/lib/Drupal/Core/Render/Element/Optgroup.php
+++ b/core/lib/Drupal/Core/Render/Element/Optgroup.php
@@ -33,6 +33,9 @@ public function getInfo() {
         array($class, 'processOptgroup'),
         array($class, 'processAjaxForm'),
       ),
+      '#pre_render' => array(
+        array($class, 'preRenderOptgroup'),
+      ),
       '#theme' => 'optgroup',
       '#options' => array(),
     );
@@ -83,7 +86,6 @@ public static function processOptgroup(&$element, FormStateInterface $form_state
         $weight += 0.001;
 
         $empty_choice = $empty_value && $key == '_none';
-        $is_default = $value_valid && ((!$value_is_array && (string) $element['#value'] === $key || ($value_is_array && in_array($key, $element['#value']))) || $empty_choice);
 
         $element += array($key => array());
         $element[$key] += array(
@@ -102,4 +104,12 @@ public static function processOptgroup(&$element, FormStateInterface $form_state
     return $element;
   }
 
+  /**
+   * Prepares an option group render element.
+   */
+  public static function preRenderOptgroup($element) {
+    $element['#attributes']['label'] = $element['#title'];
+    return $element;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Render/Element/Option.php b/core/lib/Drupal/Core/Render/Element/Option.php
index 9877d69..3251fa6 100644
--- a/core/lib/Drupal/Core/Render/Element/Option.php
+++ b/core/lib/Drupal/Core/Render/Element/Option.php
@@ -41,7 +41,7 @@ public function getInfo() {
   }
 
   /**
-   * Prepares a select render element.
+   * Prepares an option render element.
    */
   public static function preRenderOption($element) {
     $element['#attributes']['value'] = $element['#return_value'];
diff --git a/core/lib/Drupal/Core/Render/Element/Select.php b/core/lib/Drupal/Core/Render/Element/Select.php
index 08e3716..0280811 100644
--- a/core/lib/Drupal/Core/Render/Element/Select.php
+++ b/core/lib/Drupal/Core/Render/Element/Select.php
@@ -127,6 +127,45 @@ public static function processSelect(&$element, FormStateInterface $form_state,
       }
     }
 
+    // Process option groups.
+    // @TODO Fix weights with mixed optgroups and options.
+    if (count($element['#options']) > 0) {
+      // @TODO This is ported from form_select_options(). But instead of
+      //   checking #value, #default_value should be used here and below, right?
+      $value_valid = isset($element['#value']) || array_key_exists('#value', $element);
+      $value_is_array = $value_valid && is_array($element['#value']);
+      // Check if the element is multiple select and no value has been selected.
+      $empty_value = (empty($element['#value']) && !empty($element['#multiple']));
+
+      $weight = 0;
+      foreach ($element['#options'] as $key => $choice) {
+        $key = (string) $key;
+
+        if (is_array($choice)) {
+          // Maintain order of options as defined in #options, in case the element
+          // defines custom option sub-elements, but does not define all option
+          // sub-elements.
+          $weight += 0.001;
+
+          $element += array($key => array());
+          $element[$key] += array(
+            '#type' => 'optgroup',
+            '#title' => $key,
+            // The key is sanitized in Drupal\Core\Template\Attribute during output
+            // from the theme function.
+            '#options' => $choice,
+            '#ajax' => isset($element['#ajax']) ? $element['#ajax'] : NULL,
+            '#weight' => $weight,
+          );
+
+          // @TODO
+          static::processOptgroup($element[$key], $form_state, $complete_form);
+
+          unset($element['#options'][$key]);
+        }
+      }
+    }
+
     // Process the '#options' element in the same way that option groups do.
     static::processOptgroup($element, $form_state, $complete_form);