<!-- BEGIN COPY / PASTE -->
    const form = document.querySelector('form');
    form.addEventListener('submit', function(event) {
      if (!form.checkValidity()) {
        event.preventDefault(); // Prevent form submission if invalid
        alert('Please fill out all required fields correctly.');
      }
    });

    // Custom validation example
    const emailInput = document.querySelector('input[type="email"]');
    emailInput.addEventListener('input', function() {
      if (emailInput.validity.typeMismatch) {
        emailInput.setCustomValidity('Please enter a valid email address.');
      } else {
        emailInput.setCustomValidity('');
      }
    });
    <!-- END COPY / PASTE -->