Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 27368)
+++ wp-includes/media.php	(working copy)
@@ -975,9 +975,9 @@
 		if ( isset( $image_meta['height'], $image_meta['width'] ) )
 			$orientation = ( $image_meta['height'] > $image_meta['width'] ) ? 'portrait' : 'landscape';
 
-		$output .= "<{$itemtag} class='gallery-item'>";
+		$output .= "<{$itemtag} class='gallery-item' aria-describedby='$selector-$id'>";
 		$output .= "
-			<{$icontag} class='gallery-icon {$orientation}'>
+			<{$icontag} class='gallery-icon {$orientation}' id='$selector-$id'>
 				$image_output
 			</{$icontag}>";
 		if ( $captiontag && trim($attachment->post_excerpt) ) {