<!DOCTYPE html>
<html>
  <head>
    <script src="js/maze.js"></script>
    <link href="css/style.css" rel="stylesheet" type="text/css" media="screen" />
  </head>
  <body>
    <main class="container">
      <h1>Mazes</h1>
      <!-- mazes will be added here -->
    </main>
  </body>
</html>

<!DOCTYPE html>
<html>
  <head>
    <script src="js/maze.js" type="module"></script>
    <link href="css/style.css" rel="stylesheet" type="text/css" media="screen" />
  </head>
  <body>
    <main class="container">
      <h1>Mazes</h1>
      <!-- mazes will be added here -->
    </main>
  </body>
</html>

Add type=module to script tag
Add type=module to script tag
