diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index fb9fc04eef..8bbf799a82 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -1618,3 +1618,44 @@ add_action( 'admin_enqueue_scripts', '_wp_customize_loader_settings' );
function wp_customize_url( $stylesheet ) {
return esc_url( admin_url( 'customize.php' ) . '?theme=' . $stylesheet );
}
+
+/**
+ * Prints a script to check whether or not the customizer is supported,
+ * and apply either the no-customize-support or customize-support class
+ * to the body.
+ *
+ * This function MUST be called inside the body tag.
+ *
+ * Ideally, call this function immediately after the body tag is opened.
+ * This prevents a flash of unstyled content.
+ *
+ * It is also recommended that you add the "no-customize-support" class
+ * to the body tag by default.
+ *
+ * @since 3.4.0
+ */
+function wp_customize_support_script() {
+ if ( ! wp_script_is( 'customize-loader', 'queue' ) )
+ return;
+
+ $admin_origin = parse_url( admin_url() );
+ $home_origin = parse_url( home_url() );
+ $cross_domain = ( strtolower( $admin_origin[ 'host' ] ) != strtolower( $home_origin[ 'host' ] ) );
+
+ ?>
+
+