The engine is quite a large JavaScript file, so it's downloaded asynchronously in order to let the rest of the page finish loading and start being displayed. Quite straightforwardly:
<script async type="text/javascript" src="standalone-<version>.js"></script>
Make sure to replace <version> as appropriate. Bringing it all together Here's the complete skeleton web page for an HTML5 standalone:
<html> <body> <div> <canvas style="border: 0px none;" id="canvas" oncontextmenu="event.preventDefault()"></canvas> </div> <script type="text/javascript"> var Module = { canvas: document.getElementById('canvas') }; </script> <script async type="text/javascript" src="standalone-community.js"></script> </body> </html>