{% for category, message in get_flashed_messages(with_categories=True) %}
  <div class="alert alert-{{ category }}">{{ message }}</div>
{% endfor %}

<form method="post">
  <p><input name="username"></p>
  <p><input name="password" type="password"></p>
  <p><button type="submit">log in</button></p>
</form>