diff --git a/test/src4/minimal-test.html b/test/src4/minimal-test.html
index 18e69758..477a18ec 100755
--- a/test/src4/minimal-test.html
+++ b/test/src4/minimal-test.html
@@ -2,7 +2,7 @@
 @license https://github.com/t2ym/i18n-behavior/blob/master/LICENSE.md
 Copyright (c) 2019, Tetsuya Mori <t2y3141592@gmail.com>. All rights reserved.
 -->
-<html preferred>
+<html lang="en" preferred>
   <head>
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, minimum-scale=1.0, initial-scale=1.0, user-scalable=yes">
