diff --git a/core/includes/form.inc b/core/includes/form.inc
index 571a439..01e8226 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -2771,10 +2771,9 @@ function theme_fieldset($variables) {
   element_set_attributes($element, array('id'));
   _form_set_class($element, array('form-wrapper'));
 
-  $output = '<fieldset' . drupal_attributes($element['#attributes']) . '>';
+  $output = '<details' . drupal_attributes($element['#attributes']) . '>';
   if (!empty($element['#title'])) {
-    // Always wrap fieldset legends in a SPAN for CSS positioning.
-    $output .= '<legend><span class="fieldset-legend">' . $element['#title'] . '</span></legend>';
+    $output .= '<summary>' . $element['#title'] . '</summary>';
   }
   $output .= '<div class="fieldset-wrapper">';
   if (!empty($element['#description'])) {
@@ -2785,7 +2784,7 @@ function theme_fieldset($variables) {
     $output .= $element['#value'];
   }
   $output .= '</div>';
-  $output .= "</fieldset>\n";
+  $output .= "</details>\n";
   return $output;
 }