const Base64Url: brand(
  "Base64Url",
  String,
  (value: string): Result<string, Base64UrlError> => {
    // Round-trip validation ensures consistency across different base64url
    // implementations (Node.js Buffer, native browser API, manual fallback).
    // Only strings that decode and encode identically are accepted.
    let roundTrip;
    try {
      roundTrip = uint8ArrayToBase64Url(
        base64UrlToUint8Array(value as Base64Url),
      );
    } catch {
      //
    }
    return roundTrip === value
      ? ok(value)
      : err<Base64UrlError>({ type: "Base64Url", value });
  },
);