diff --git a/testing/web-platform/tests/interfaces/native-file-system.idl b/testing/web-platform/tests/interfaces/native-file-system.idl
index f4679175c46ec..60a28f055ded9 100644
--- a/testing/web-platform/tests/interfaces/native-file-system.idl
+++ b/testing/web-platform/tests/interfaces/native-file-system.idl
@@ -13,18 +13,20 @@ interface FileSystemHandle {
   readonly attribute boolean isDirectory;
   readonly attribute USVString name;
 
+  Promise<boolean> isSameEntry(FileSystemHandle other);
+
   Promise<PermissionState> queryPermission(optional FileSystemHandlePermissionDescriptor descriptor = {});
   Promise<PermissionState> requestPermission(optional FileSystemHandlePermissionDescriptor descriptor = {});
 };
 
-dictionary FileSystemCreateWriterOptions {
+dictionary FileSystemCreateWritableOptions {
   boolean keepExistingData = false;
 };
 
 [Exposed=(Window,Worker), SecureContext, Serializable]
 interface FileSystemFileHandle : FileSystemHandle {
   Promise<File> getFile();
-  Promise<FileSystemWriter> createWriter(optional FileSystemCreateWriterOptions options = {});
+  Promise<FileSystemWritableFileStream> createWritable(optional FileSystemCreateWritableOptions options = {});
 };
 
 dictionary FileSystemGetFileOptions {
@@ -41,20 +43,36 @@ dictionary FileSystemRemoveOptions {
 
 [Exposed=(Window,Worker), SecureContext, Serializable]
 interface FileSystemDirectoryHandle : FileSystemHandle {
+  async iterable<USVString, FileSystemHandle>;
+
   Promise<FileSystemFileHandle> getFile(USVString name, optional FileSystemGetFileOptions options = {});
   Promise<FileSystemDirectoryHandle> getDirectory(USVString name, optional FileSystemGetDirectoryOptions options = {});
 
-  // This really returns an async iterable, but that is not yet expressable in WebIDL.
-  object getEntries();
-
   Promise<void> removeEntry(USVString name, optional FileSystemRemoveOptions options = {});
+
+  Promise<sequence<USVString>?> resolve(FileSystemHandle possibleDescendant);
 };
 
+enum WriteCommandType {
+  "write",
+  "seek",
+  "truncate",
+};
+
+dictionary WriteParams {
+  required WriteCommandType type;
+  unsigned long long? size;
+  unsigned long long? position;
+  (BufferSource or Blob or USVString)? data;
+};
+
+typedef (BufferSource or Blob or USVString or WriteParams) FileSystemWriteChunkType;
+
 [Exposed=(Window,Worker), SecureContext]
-interface FileSystemWriter {
-  Promise<void> write(unsigned long long position, (BufferSource or Blob or USVString) data);
+interface FileSystemWritableFileStream : WritableStream {
+  Promise<void> write(FileSystemWriteChunkType data);
+  Promise<void> seek(unsigned long long position);
   Promise<void> truncate(unsigned long long size);
-  Promise<void> close();
 };
 
 enum ChooseFileSystemEntriesType { "open-file", "save-file", "open-directory" };
@@ -78,15 +96,7 @@ partial interface Window {
         chooseFileSystemEntries(optional ChooseFileSystemEntriesOptions options = {});
 };
 
-enum SystemDirectoryType {
-  "sandbox"
-};
-
-dictionary GetSystemDirectoryOptions {
-  required SystemDirectoryType type;
-};
-
 [SecureContext]
-partial interface FileSystemDirectoryHandle {
-  static Promise<FileSystemDirectoryHandle> getSystemDirectory(GetSystemDirectoryOptions options);
+partial interface mixin WindowOrWorkerGlobalScope {
+  Promise<FileSystemDirectoryHandle> getOriginPrivateDirectory();
 };
diff --git a/testing/web-platform/tests/native-file-system/idlharness.https.any.js b/testing/web-platform/tests/native-file-system/idlharness.https.any.js
index 98b27fd9e3842..a9090baeee5a9 100644
--- a/testing/web-platform/tests/native-file-system/idlharness.https.any.js
+++ b/testing/web-platform/tests/native-file-system/idlharness.https.any.js
@@ -6,7 +6,7 @@
 
 idl_test(
   ['native-file-system'],
-  ['html', 'dom'],
+  ['streams', 'html', 'dom'],
   idl_array => {
     idl_array.add_objects({
       // TODO: Add instances of FileSystemHandle, FileSystemFileHandle,