Comment form vaildation

Ok, I’ve worked this out. You need to copy the complete first section of the `comment_form()’ from wp core. This is the code

<?php
function comment_form( $args = array(), $post_id = null ) {
    if ( null === $post_id )
        $post_id = get_the_ID();
    else
        $id = $post_id;

    $commenter = wp_get_current_commenter();
    $user = wp_get_current_user();
    $user_identity = $user->exists() ? $user->display_name : '';

    $args = wp_parse_args( $args );
    if ( ! isset( $args['format'] ) )
        $args['format'] = current_theme_supports( 'html5', 'comment-form' ) ? 'html5' : 'xhtml';

    $req      = get_option( 'require_name_email' );
    $aria_req = ( $req ? " aria-required='true'" : '' );
    $html5    = 'html5' === $args['format'];
    $fields   =  array(
        'author' => '<p class="comment-form-author">' . '<label for="author">' . __( 'Name' ) . ( $req ? ' <span class="required">*</span>' : '' ) . '</label> ' .
                    '<input id="author" name="author" type="text" value="' . esc_attr( $commenter['comment_author'] ) . '" size="30"' . $aria_req . ' /></p>',
        'email'  => '<p class="comment-form-email"><label for="email">' . __( 'Email' ) . ( $req ? ' <span class="required">*</span>' : '' ) . '</label> ' .
                    '<input id="email" name="email" ' . ( $html5 ? 'type="email"' : 'type="text"' ) . ' value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30"' . $aria_req . ' /></p>',
        'url'    => '<p class="comment-form-url"><label for="url">' . __( 'Website' ) . '</label> ' .
                    '<input id="url" name="url" ' . ( $html5 ? 'type="url"' : 'type="text"' ) . ' value="' . esc_attr( $commenter['comment_author_url'] ) . '" size="30" /></p>',
    );

    $required_text = sprintf( ' ' . __('Required fields are marked %s'), '<span class="required">*</span>' );

    /**
     * Filter the default comment form fields.
     *
     * @since 3.0.0
     *
     * @param array $fields The default comment fields.
     */
    $fields = apply_filters( 'comment_form_default_fields', $fields );
    $defaults = array(
        'fields'               => $fields,
        'comment_field'        => '<p class="comment-form-comment"><label for="comment">' . _x( 'Comment', 'noun' ) . '</label> <textarea id="comment" name="comment" cols="45" rows="8" aria-required="true"></textarea></p>',
        'must_log_in'          => '<p class="must-log-in">' . sprintf( __( 'You must be <a href="https://wordpress.stackexchange.com/questions/102148/%s">logged in</a> to post a comment.' ), wp_login_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ) . '</p>',
        'logged_in_as'         => '<p class="logged-in-as">' . sprintf( __( 'Logged in as <a href="%1$s">%2$s</a>. <a href="%3$s" title="Log out of this account">Log out?</a>' ), get_edit_user_link(), $user_identity, wp_logout_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ) . '</p>',
        'comment_notes_before' => '<p class="comment-notes">' . __( 'Your email address will not be published.' ) . ( $req ? $required_text : '' ) . '</p>',
        'comment_notes_after'  => '<p class="form-allowed-tags">' . sprintf( __( 'You may use these <abbr title="HyperText Markup Language">HTML</abbr> tags and attributes: %s' ), ' <code>' . allowed_tags() . '</code>' ) . '</p>',
        'id_form'              => 'commentform',
        'id_submit'            => 'submit',
        'title_reply'          => __( 'Leave a Reply' ),
        'title_reply_to'       => __( 'Leave a Reply to %s' ),
        'cancel_reply_link'    => __( 'Cancel reply' ),
        'label_submit'         => __( 'Post Comment' ),
        'format'               => 'xhtml',
    );
?>

You can now make your changes, rename the function, and add it to comment_form_defaults

Even if you need to just change one thing, you will need to cope this complete section for the validation to work correctly