Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/system/templates/progress-bar.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation for a progress bar.
+ *
+ * Note that the core Batch API uses this only for non-JavaScript batch jobs.
+ *
+ * Available variables:
+ * - label: The label of the working task.
+ * - percent: The percentage of the progress.
+ * - message: A string containing information to be displayed.
+ *
+ * @ingroup themeable
+ */
+#}
+<div class="progress" data-drupal-progress>
+  {% if label %}
+    <div class="progress__label">{{ label }}</div>
+  {% endif %}
+  <div class="progress__track"><div class="progress__bar" style="width: {{ percent }}%"></div></div>
+  <div class="progress__percentage">{{ percent }}%</div>
+  <div class="progress__description">{{ message }}</div>
+</div>