diff --git a/wp-admin/admin-header.php b/wp-admin/admin-header.php
index b3316c2c0f..3b52e3acca 100644
--- a/wp-admin/admin-header.php
+++ b/wp-admin/admin-header.php
@@ -93,7 +93,16 @@ if ( wp_is_mobile() )
 ?>
 </head>
 <body class="wp-admin no-js <?php echo apply_filters( 'admin_body_class', '' ) . " $admin_body_class"; ?>">
-<script type="text/javascript">document.body.className = document.body.className.replace('no-js','js');</script>
+<script type="text/javascript">
+	document.body.className = document.body.className.replace('no-js','js');
+<?php
+// If the customize loader is enqueued, then add the 'customize-support' class early.
+// This prevents a flash of unstyled content.
+if ( wp_script_is( 'customize-loader', 'queue' ) ) : ?>
+	if ( window.postMessage )
+		document.body.className += ' customize-support';
+<?php endif; ?>
+</script>
 
 <div id="wpwrap">
 <?php require(ABSPATH . 'wp-admin/menu-header.php'); ?>
