xhr/sync.html

<!DOCTYPE html>
<html>
  <head>
    <style type="text/css">
      body {
        font-family: sans-serif;
      }
      .panel {
        border: 1px solid red
      }
      .container {
        display: flex
      }
      .container > * {
        flex: 0 0 50%;
        height: 100%
      }
    </style>
    <meta charset="utf-8">
    <title>AJAX SYNC</title>
  </head>
  <body>
    <h1>My Application</h1>
    <div class="msg"></div>
    <button type="button" name="button">Show OK</button>
    <div class="container">
      <div id="panel-1" class="panel">Not started</div>
      <div id="panel-2" class="panel"></div>
    </div>
    <script type="text/javascript" src="sync.js"></script>
  </body>

</html>