Edit in JSFiddle

document.write('¡Hola mundo!');
<!DOCTYPE html>
<html>

  <head>
    <meta charset="utf-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>Page Title</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <script src="main.js"></script>
  </head>

  <body>

  </body>

</html>