<div class="header">
  <a href="/">Home</a>
  <button popovertarget="pop">Login</button>
</div>

<aside id="pop" popover="manual">
  <div>
    <h2>Login</h2>
    <form>
      <input type="email" placeholder="Email">
      <input type="password" placeholder="Password">
      <button type="submit">Login</button>
    </form>
    <p>Don't have an account? <a href="#">Register</a></p>
  </div>
</aside>