It seems WP core adds the novlidate attribute to the comment form when your theme has HTML5 support enabled for comment forms (see includes/comment-template.php).
To disable it, use
remove_theme_support('html5', 'comment-form');
It seems WP core adds the novlidate attribute to the comment form when your theme has HTML5 support enabled for comment forms (see includes/comment-template.php).
To disable it, use
remove_theme_support('html5', 'comment-form');