diff --git a/wp-content/themes/twentyfifteen/functions.php b/wp-content/themes/twentyfifteen/functions.php
index 61dfdcf446..f0d94ecc53 100644
--- a/wp-content/themes/twentyfifteen/functions.php
+++ b/wp-content/themes/twentyfifteen/functions.php
@@ -193,6 +193,18 @@ function twentyfifteen_fonts_url() {
 }
 endif;
 
+/**
+ * JavaScript Detection.
+ *
+ * Adds a `js` class to the root `<html>` element when JavaScript is detected.
+ *
+ * @since Twenty Fifteen 1.1
+ */
+function twentyfifteen_javascript_detection() {
+	echo "<script>(function(html){html.className = html.className.replace(/\bno-js\b/,'js')})(document.documentElement);</script>\n";
+}
+add_action( 'wp_head', 'twentyfifteen_javascript_detection', 0 );
+
 /**
  * Enqueue scripts and styles.
  *
diff --git a/wp-content/themes/twentyfifteen/header.php b/wp-content/themes/twentyfifteen/header.php
index d8e387ff7a..e415915c59 100644
--- a/wp-content/themes/twentyfifteen/header.php
+++ b/wp-content/themes/twentyfifteen/header.php
@@ -18,7 +18,6 @@
 	<!--[if lt IE 9]>
 	<script src="<?php echo esc_url( get_template_directory_uri() ); ?>/js/html5.js"></script>
 	<![endif]-->
-	<script>(function(){document.documentElement.className='js'})();</script>
 	<?php wp_head(); ?>
 </head>
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 6566c66f92..2c0b7fb346 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-alpha-31183';
+$wp_version = '4.2-alpha-31184';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
