TIL; check validity & report validity
No need to call form.checkValidity() after a form has been submitted. That
verification has already been performed on submission.
In a React component, the function passed to onSubmit attribute is only
executed when the form is valid. Errors are shown to the user as a tooltip
otherwise.
That being said, it's still possible to use element.reportValidity() to force
the browser to perform this verification, without submitting the form.