Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 24502)
+++ wp-includes/comment-template.php	(working copy)
@@ -1629,6 +1629,7 @@
 		'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 __( 'Your email address will not be published.'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>',
+		'class_form'           => '',
 		'id_form'              => 'commentform',
 		'id_submit'            => 'submit',
 		'title_reply'          => __( 'Leave a Reply' ),
@@ -1649,7 +1650,7 @@
 					<?php echo $args['must_log_in']; ?>
 					<?php do_action( 'comment_form_must_log_in_after' ); ?>
 				<?php else : ?>
-					<form action="<?php echo site_url( '/wp-comments-post.php' ); ?>" method="post" id="<?php echo esc_attr( $args['id_form'] ); ?>"<?php echo $html5 ? ' novalidate' : ''; ?>>
+					<form action="<?php echo site_url( '/wp-comments-post.php' ); ?>" method="post"<?php echo ( $args['class_form'] ? ' class="' . esc_attr( $args['class_form'] ) . '"' : '' ); ?> id="<?php echo esc_attr( $args['id_form'] ); ?>"<?php echo $html5 ? ' novalidate' : ''; ?>>
 						<?php do_action( 'comment_form_top' ); ?>
 						<?php if ( is_user_logged_in() ) : ?>
 							<?php echo apply_filters( 'comment_form_logged_in', $args['logged_in_as'], $commenter, $user_identity ); ?>