{% if error %}
  <p style="color:red">{{ error }}</p>
{% endif %}

<table>
 <caption>Регистрация</caption>
 <form method="post">
   <tr><th><label>Имя:</th><td><input type="text" name="username" required></label></td></tr>
   <tr><th><label>Email:</th><td><input type="email" name="mail" required></label></td></tr>
   <tr><th><label>Пароль:</th><td><input type="password" name="password" required></label></td></tr>
   <tr><th colspan="2"><button type="submit">Зарегистрироваться</button></th></tr>
 </form>
</table>

<hr>Есть учетка: <a href="/login">Войти</a>