Is there a browser-side validation passed event I can listen for?
Drupal doesn't have a browser-side validation framework as it does with the back-end form validation handlers. The &quot;Please fill...&quot; message is the browser enforcing the required property.
If you'd like to hook into those validation events, you can bind event listeners on specific form elements for invalid events. If you want to take action on a submit click, the checkValidity() method can be executed on form field DOM objects.
See the MDN docs on the constraint validation API for more info.