index.html

<!doctype html>
<html>
  <head>
    <title>MyLib Demo</title>

    <link
      href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.1/dist/css/bootstrap.min.css"
      rel="stylesheet"
      integrity="sha384-4bw+/aepP/YC94hEpVNVgiZdgIC5+VKNBQNGCHeKRQN+PtmoHDEXuppvnDJzQIu9"
      crossorigin="anonymous"
    />
  </head>

  <body>
    <div class="container py-5">
      <h1 class="mb-4">MyLib Demo</h1>

      <p id="esm" class="text-success"></p>
      <p id="umd" class="text-success"></p>
    </div>

    <script type="module">
      import foo from "./src/index.ts"

      document.querySelector("#esm").innerHTML = foo("esm")
    </script>

    <script src="./dist/index.umd.js"></script>
    <script>
      document.querySelector("#umd").innerHTML = myLib("umd")
    </script>
  </body>
</html>