gistfile1.html

<style>
  .sign-in {
    border: 1px solid black;
  }

  .sign-in.is-unavailable {
    background-color: #eee;
  }
</style>

<main class=“sign-in js-sign-in”>
</main>

<script>
  document.querySelector('.js-sign-in').classList.add('is-unavailable');
</script>