+ * If a "data" key is present, it must be an array, where the keys represent
+ * the data-xxx properties with their provided values. For example,
+ * $item['data'] = array(
+ * 'foo' => 1,
+ * 'bar' => 'baz',
+ * );
+ * will render as element properties:
+ * data-foo='1' data-bar='baz'
+ *