diff --git a/index.htm b/index.htm
index 835fa0f..0e06641 100644
--- a/index.htm
+++ b/index.htm
@@ -130,8 +130,8 @@ By <a href="mailto:inexorabletash@gmail.com">Joshua Bell</a>
 <div id="paper"></div>
 
 <!-- CodeMirror syntax highlighting - this is optional -->
-<script src="//cdnjs.cloudflare.com/ajax/libs/codemirror/3.20.0/codemirror.js"></script>
-<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/codemirror/3.20.0/codemirror.css">
+<script src="//cdnjs.cloudflare.com/ajax/libs/codemirror/4.0.3/codemirror.js"></script>
+<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/codemirror/4.0.3/codemirror.css">
 <script src="cm/basic.js"></script>
 <link rel="stylesheet" href="cm/basic.css">
 <style type="text/css">