diff --git a/testing/web-platform/tests/interfaces/streams.idl b/testing/web-platform/tests/interfaces/streams.idl
index 35f558f624a8..3d7484a20e05 100644
--- a/testing/web-platform/tests/interfaces/streams.idl
+++ b/testing/web-platform/tests/interfaces/streams.idl
@@ -137,7 +137,7 @@ dictionary UnderlyingSink {
 };
 
 callback UnderlyingSinkStartCallback = any (WritableStreamDefaultController controller);
-callback UnderlyingSinkWriteCallback = Promise<void> (WritableStreamDefaultController controller, optional any chunk);
+callback UnderlyingSinkWriteCallback = Promise<void> (any chunk, WritableStreamDefaultController controller);
 callback UnderlyingSinkCloseCallback = Promise<void> ();
 callback UnderlyingSinkAbortCallback = Promise<void> (optional any reason);
 
@@ -180,7 +180,7 @@ dictionary Transformer {
 
 callback TransformerStartCallback = any (TransformStreamDefaultController controller);
 callback TransformerFlushCallback = Promise<void> (TransformStreamDefaultController controller);
-callback TransformerTransformCallback = Promise<void> (TransformStreamDefaultController controller, optional any chunk);
+callback TransformerTransformCallback = Promise<void> (any chunk, TransformStreamDefaultController controller);
 
 [Exposed=(Window,Worker,Worklet)]
 interface TransformStreamDefaultController {