--- a/testing/web-platform/tests/interfaces/encoding.idl
+++ b/testing/web-platform/tests/interfaces/encoding.idl
@@ -13,20 +13,20 @@ dictionary TextDecoderOptions {
   boolean fatal = false;
   boolean ignoreBOM = false;
 };
 
 dictionary TextDecodeOptions {
   boolean stream = false;
 };
 
-[Constructor(optional DOMString label = "utf-8", optional TextDecoderOptions options),
+[Constructor(optional DOMString label = "utf-8", optional TextDecoderOptions options = {}),
  Exposed=(Window,Worker)]
 interface TextDecoder {
-  USVString decode(optional BufferSource input, optional TextDecodeOptions options);
+  USVString decode(optional BufferSource input, optional TextDecodeOptions options = {});
 };
 TextDecoder includes TextDecoderCommon;
 
 interface mixin TextEncoderCommon {
   readonly attribute DOMString encoding;
 };
 
 dictionary TextEncoderEncodeIntoResult {