Put it all together and see if anything works.

index.html

@@ -3,8 +3,13 @@
   <head>
     <meta charset="utf-8">
     <title>This page is powered by Markdown.js</title>
+    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/showdown/1.8.6/showdown.min.js">
+    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js">
   </head>
   <body>
     <h1>Hello World! (for now ...)</h1>
+    <div id="markdown-content">TODO</div>
+    <!-- That is all we need, now load the script and get going! -->
+    <script type="text/javascript" src="use-markdown.js"></script>
   </body>
 </html>