Revise the Zebra variable declare in the template preprocessor

core/includes/theme.inc

@@ -1081,6 +1081,14 @@ function template_preprocess_maintenance_task_list(&$variables) {
  * details.
  */
 function template_preprocess(&$variables, $hook, $info) {
+  $count = [];
+
+  // Track run count for each hook to provide zebra striping. See
+  // "template_preprocess_block()" which provides the same feature specific to
+  // blocks.
+  $count[$hook] = isset($count[$hook]) && is_int($count[$hook]) ? $count[$hook] : 1;
+  $variables['zebra'] = ($count[$hook] % 2) ? 'odd' : 'even';
+
   // Merge in variables that don't depend on hook and don't change during a
   // single page request.
   // Use the advanced drupal_static() pattern, since this is called very often.

core/modules/node/templates/node.html.twig

@@ -54,6 +54,8 @@
  *   displayed after the main title tag that appears in the template.
  * - view_mode: View mode; for example, "teaser" or "full".
  * - page: Flag for the full page state. Will be true if view_mode is 'full'.
+ * - zebra: Outputs either "even" or "odd". Useful for zebra striping in
+ *   teaser listings.
  *
  * @see template_preprocess_node()
  *