Index: src/wp-includes/general-template.php
===================================================================
--- src/wp-includes/general-template.php	(revision 50565)
+++ src/wp-includes/general-template.php	(working copy)
@@ -3687,7 +3687,7 @@
 		),
 		'htmlhint'   => array(
 			'tagname-lowercase'        => true,
-			'attr-lowercase'           => true,
+			'attr-lowercase'           => false,
 			'attr-value-double-quotes' => false,
 			'doctype-first'            => false,
 			'tag-pair'                 => true,