MAGETWO-98765: Apply HTML5 Attributes to Newsletter Preview

Author: omiroshnichenko

app/code/Magento/Newsletter/view/adminhtml/templates/preview/iframeswitcher.phtml

@@ -15,7 +15,13 @@
         </div>
         <?php endif;?>
     </div>
-    <iframe name="preview_iframe" id="preview_iframe" frameborder="0" title="<?php /* @escapeNotVerified */ echo __('Preview') ?>" width="100%"></iframe>
+    <iframe name="preview_iframe"
+            id="preview_iframe"
+            frameborder="0"
+            title="<?php /* @escapeNotVerified */ echo __('Preview') ?>"
+            width="100%"
+            sandbox="allow-forms allow-pointer-lock allow-scripts">
+    </iframe>
     <?php echo $block->getChildHtml('preview_form'); ?>
 </div>