form.js

const form = document.querySelector('form');
const mail = document.getElementById('mail');
const emailRegExp = /^[a-z-0-9.!#$%&'*+/=?^_`{|}~-]+@[a-z-0-9-]+(?:\.[a-z-0-9-]+)*$/;
const warning = mail.nextElementSibling;
form.addEventListener('submit', (event) => {
  event.preventDefault();

  const isValid = mail.value.length === 0 || emailRegExp.test(mail.value);
  if (!isValid) {
    mail.className = 'invalid';
    warning.textContent = 'Please enter a valid email address" The email must not contain capital letters. (Example of valid email: pierjaeab@gmail.com)';
    warning.className = 'warning active';
  } else {
    mail.className = 'valid';
    warning.textContent = '';
    warning.className = 'warning';
    form.submit();
  }
});