code: public static String filterNormalizeUri(SoyValue value) {
      value = normalizeNull(value);
      if (isSanitizedContentOfKind(value, SanitizedContent.ContentKind.URI)
          || isSanitizedContentOfKind(value, SanitizedContent.ContentKind.TRUSTED_RESOURCE_URI)) {
        return normalizeUri(value);
      }
      return filterNormalizeUri(value.coerceToString());
    }
docs: Makes sure that the given input doesn't specify a dangerous protocol and also {@link
  #normalizeUri normalizes} it.