Index: wp-content/themes/twentythirteen/404.php
===================================================================
--- wp-content/themes/twentythirteen/404.php	(revision 23793)
+++ wp-content/themes/twentythirteen/404.php	(working copy)
@@ -21,7 +21,7 @@
 					<h2><?php _e( 'This is somewhat embarrassing, isn&rsquo;t it?', 'twentythirteen' ); ?></h2>
 					<p><?php _e( 'It looks like nothing was found at this location. Maybe try a search?', 'twentythirteen' ); ?></p>
 
-					<?php get_search_form( true, 'html5' ); ?>
+					<?php get_search_form(); ?>
 				</div><!-- .page-content -->
 			</div><!-- .page-wrapper -->
 
Index: wp-content/themes/twentythirteen/content-none.php
===================================================================
--- wp-content/themes/twentythirteen/content-none.php	(revision 23793)
+++ wp-content/themes/twentythirteen/content-none.php	(working copy)
@@ -20,12 +20,12 @@
 	<?php elseif ( is_search() ) : ?>
 
 	<p><?php _e( 'Sorry, but nothing matched your search terms. Please try again with different keywords.', 'twentythirteen' ); ?></p>
-	<?php get_search_form( true, 'html5' ); ?>
+	<?php get_search_form(); ?>
 
 	<?php else : ?>
 
 	<p><?php _e( 'It seems we can&rsquo;t find what you&rsquo;re looking for. Perhaps searching can help.', 'twentythirteen' ); ?></p>
-	<?php get_search_form( true, 'html5' ); ?>
+	<?php get_search_form(); ?>
 
 	<?php endif; ?>
 </div><!-- .page-content -->
Index: wp-content/themes/twentythirteen/header.php
===================================================================
--- wp-content/themes/twentythirteen/header.php	(revision 23793)
+++ wp-content/themes/twentythirteen/header.php	(working copy)
@@ -51,7 +51,7 @@
 					<h3 class="menu-toggle"><?php _e( 'Menu', 'twentythirteen' ); ?></h3>
 					<a class="screen-reader-text skip-link" href="#content" title="<?php esc_attr_e( 'Skip to content', 'twentythirteen' ); ?>"><?php _e( 'Skip to content', 'twentythirteen' ); ?></a>
 					<?php wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu' ) ); ?>
-					<?php get_search_form( true, 'html5' ); ?>
+					<?php get_search_form(); ?>
 				</nav><!-- #site-navigation -->
 			</div><!-- #navbar -->
 		</header><!-- #masthead -->
Index: wp-content/themes/twentythirteen/functions.php
===================================================================
--- wp-content/themes/twentythirteen/functions.php	(revision 23793)
+++ wp-content/themes/twentythirteen/functions.php	(working copy)
@@ -548,6 +548,17 @@
 add_filter( 'the_content', 'twentythirteen_aside_date', 8 ); // After embeds, before everything else.
 
 /**
+ * Switches default core markup for search form to output valid HTML5.
+ *
+ * @param string $format Expected markup format, default is `xhtml`
+ * @return string Twenty Thirteen loves HTML5.
+ */
+function twentythirteen_searchform_format( $format ) {
+	return 'html5';
+}
+add_filter( 'search_form_format', 'twentythirteen_searchform_format' );
+
+/**
  * Add postMessage support for site title and description for the Customizer.
  *
  * @since Twenty Thirteen 1.0