Index: src/wp-includes/comment-template.php
===================================================================
--- src/wp-includes/comment-template.php	(revision 40900)
+++ src/wp-includes/comment-template.php	(working copy)
@@ -2273,9 +2273,10 @@
 	 * @since 3.0.0
 	 */
 	do_action( 'comment_form_before' );
-	?>
-	<div id="respond" class="comment-respond">
-		<?php
+	$respond_class = 'comment-respond';
+	$respond_class = apply_filters ( 'comment_form_respond_class' ,$respond_class );
+	echo '<div id="respond" class="' . esc_attr( $respond_class ) . '">';
+	
 		echo $args['title_reply_before'];
 
 		comment_form_title( $args['title_reply'], $args['title_reply_to'] );