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

core/modules/rdf/templates/rdf-metadata.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation for empty spans with RDF attributes.
+ *
+ * The XHTML+RDFa doctype allows either <span></span> or <span /> syntax to
+ * be used, but for maximum browser compatibility, W3C recommends the
+ * former when serving pages using the text/html media type, see
+ * http://www.w3.org/TR/xhtml1/#C_3.
+ *
+ * Available variables:
+ * - metadata: Each item within corresponds to its own set of attributes,
+ *   and therefore, needs its own 'attributes' element.
+ *
+ * @see template_preprocess_rdf_metadata()
+ *
+ * @ingroup themeable
+ */
+#}
+{% for attributes in metadata %}
+  <span{{ attributes.addClass('hidden') }}></span>
+{% endfor %}