index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <link rel="stylesheet" href="index.css" />
    <title>Document</title>
  </head>
  <body>
    <textarea type="text" name="data" id="data_input" markdown></textarea>
    <button onclick="getTheInput()">Download</button>
    <script src="index.js"></script>
  </body>
</html>