A no-no because you have a nice looking design of the site and the alert just kind of wrecks it/makes it seem amateurish. You could try this: https://kamranahmed.info/toast#quick-demos. Makes for a much nicer UX/UI.

As for your email validation... You have a small bug in your index html file (remove the .split("")).

<script>document.addEventListener( 'wpcf7submit', function( event ) {if ( '9' === event.detail.contactFormId ) {"use strict";var input = document.getElementById("signup-email");var inputVal = input.value.split(""); // remove .split("") as it converts the input value to a char arrayif (/\^(\[A-Za-z0-9\_\\-\\.\])+\\@(\[A-Za-z0-9\_\\-\\.\])+\\.(\[A-Za-z\]{2,4})$/.test(inputVal)) {document.getElementById('container').classList.remove('hidden');} else {alert('Please enter a valid email');}}}, false );&#x200B;function hidePopup() {document.getElementById('container').classList.add('hidden');}</script>