add_action( 'comment_form', function() {
    ?>
    <script>
    (function() {
        const form = document.querySelector('#commentform');
        if ( typeof form === 'undefined' ) return;

        const textArea = form.querySelector('textarea#comment');

        if ( form.hasAttribute('novalidate') ) {
            form.removeAttribute('novalidate');
        }
        if ( ! form.hasAttribute('validate') ) {
            form.setAttribute('validate', '');
        }

        if ( typeof textArea !== 'undefined' ) {
            textArea.setAttribute('minlength', '20');
            textArea.setAttribute('maxlength', '2000');
        }
    })();
    </script>
    <?php
}, 10, 0 );