index.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <title>React Simple</title>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.34/browser.js"></script>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.34/browser-polyfill.js"></script>
    <script type="text/javascript" src="https://fb.me/react-0.14.7.js"></script>
    <script type="text/javascript" src="https://fb.me/react-dom-0.14.7.js"></script>
    <script type="text/javascript">
      window.App = {}
    </script>
  </head>
  <body>
    <div id="app"></div>
    <script type="text/babel" src="./components/HelloWorld.js"></script>
    <script type="text/babel" src="./index.js"></script>
  </body>
</html>