index.html

<!doctype html>
<html>
<head>

  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">

  <title>paper-date-picker</title>

  <script src="../@webcomponents/webcomponentsjs/webcomponents-bundle.js"></script>
  <script type="module" src="../@polymer/iron-component-page/iron-component-page.js"></script>

</head>
<body>

  <iron-component-page></iron-component-page>

</body>