diff --git a/core/includes/common.inc b/core/includes/common.inc
index 14154af..ef2c0d7 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -4035,6 +4035,8 @@ function drupal_region_class($region) {
  *       which drupal_add_js() happened earlier in the page request.
  *   - defer: If set to TRUE, the defer attribute is set on the <script>
  *     tag. Defaults to FALSE.
+ *   - async: If set to TRUE, the async attribute is set on the <script>
+ *     tag. Defaults to FALSE.
  *   - cache: If set to FALSE, the JavaScript file is loaded anew on every page
  *     call; in other words, it is not cached. Used only when 'type' references
  *     a JavaScript file. Defaults to TRUE.
@@ -4107,6 +4109,7 @@ function drupal_add_js($data = NULL, $options = NULL) {
           'preprocess' => TRUE,
           'cache' => TRUE,
           'defer' => FALSE,
+          'async' => FALSE,
           'browsers' => array(),
         ),
       );
@@ -4154,6 +4157,7 @@ function drupal_js_defaults($data = NULL) {
     'scope' => 'header',
     'cache' => TRUE,
     'defer' => FALSE,
+    'async' => FALSE,
     'preprocess' => TRUE,
     'version' => NULL,
     'data' => $data,
@@ -4321,9 +4325,6 @@ function drupal_pre_render_scripts($elements) {
       foreach ($group['items'] as $item) {
         // Element properties that do not depend on item type.
         $element = $element_defaults;
-        if (!empty($item['defer'])) {
-          $element['#attributes']['defer'] = 'defer';
-        }
         $element['#browsers'] = $item['browsers'];
 
         // Element properties that depend on item type.
@@ -4351,6 +4352,18 @@ function drupal_pre_render_scripts($elements) {
             break;
         }
 
+        // The defer and async attributes must not be specified if the src
+        // attribute is not present.
+        if (!empty($element['#attributes']['src'])) {
+          // Both may be specified for legacy browser fallback purposes.
+          if (!empty($item['async'])) {
+            $element['#attributes']['async'] = 'async';
+          }
+          if (!empty($item['defer'])) {
+            $element['#attributes']['defer'] = 'defer';
+          }
+        }
+
         $elements[] = $element;
       }
     }