index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <!-- CSS Link -->
    <link rel="stylesheet" href="style.css" />
    <title>HTML, CSS and JavaScript Code Editor</title>
  </head>
  <body spellcheck="false">
    <textarea id="html" placeholder="HTML"></textarea>
    <textarea id="css" placeholder="CSS"></textarea>
    <textarea id="js" placeholder="JavaScript"></textarea>
    <iframe id="code"></iframe>

    <script type="text/javascript" src="script.js"></script>
  </body>
</html>