Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/lib/Drupal/Component/Utility/Xss.php

@@ -224,6 +224,8 @@ class Xss {
             $skip_protocol_filtering = substr($attribute_name, 0, 5) === 'data-' || in_array($attribute_name, array(
               'title',
               'alt',
+              'rel',
+              'property',
             ));
 
             $working = $mode = 1;