Index: wp-content/themes/twentyeleven/header.php
===================================================================
--- wp-content/themes/twentyeleven/header.php	(revision 17982)
+++ wp-content/themes/twentyeleven/header.php	(working copy)
@@ -19,7 +19,7 @@
 <html id="ie8" <?php language_attributes(); ?>>
 <![endif]-->
 <!--[if !(IE 6) | !(IE 7) | !(IE 8)  ]><!-->
-<html <?php language_attributes(); ?>>
+<html xmlns="http://www.w3.org/1999/xhtml" <?php language_attributes(); ?>>
 <!--<![endif]-->
 <head>
 <meta charset="<?php bloginfo( 'charset' ); ?>" />
Index: wp-content/themes/twentyten/header.php
===================================================================
--- wp-content/themes/twentyten/header.php	(revision 17982)
+++ wp-content/themes/twentyten/header.php	(working copy)
@@ -9,7 +9,7 @@
  * @since Twenty Ten 1.0
  */
 ?><!DOCTYPE html>
-<html <?php language_attributes(); ?>>
+<html xmlns="http://www.w3.org/1999/xhtml" <?php language_attributes(); ?>>
 <head>
 <meta charset="<?php bloginfo( 'charset' ); ?>" />
 <title><?php
