diff --git a/dev/a11y_assessments/web/index.html b/dev/a11y_assessments/web/index.html
index b3ce3240ca..84d4840c14 100644
--- a/dev/a11y_assessments/web/index.html
+++ b/dev/a11y_assessments/web/index.html
@@ -3,7 +3,7 @@
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file. -->
 
-<html lang="en_US">
+<html lang="en-US">
 <head>
   <!--
     If you are serving your web app in a path other than the root, change the
