e and specified attributes.
 *
 * This will allow the elements as custom tags (such as &lt;x-foo&gt;&lt;/x-foo&gt;),
 * but will not allow tag extensions. Use [allowTagExtension] to allow
 * tag extensions.
 */
void allowCustomElement(String tagName,
    {UriPolicy uriPolicy,
    Iterable&lt;String&gt; attributes,
    Iterable&lt;String&gt; uriAttributes}) {

  var tagNameUpper = tagName.toUpperCase();
  var attrs;
  if (attributes != null) {
    attrs =
        attributes.map((name) =&gt; &apos;$tagNameUpp