diff -u b/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
--- b/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -260,9 +260,9 @@
         'sidebar_first' => 'Left sidebar',
         'sidebar_second' => 'Right sidebar',
         'content' => 'Content',
-		'header_first' => 'Header first',
-		'header_second' => 'Header second',
-		'header_third' => 'Header third',
+        'header_first' => 'Header first',
+        'header_second' => 'Header second',
+        'header_third' => 'Header third',
         'footer' => 'Footer',
         'highlighted' => 'Highlighted',
         'help' => 'Help',