diff --git a/api-reports/2_12.txt b/api-reports/2_12.txt
index bf03dfdaa..8fbe43785 100644
--- a/api-reports/2_12.txt
+++ b/api-reports/2_12.txt
@@ -2293,6 +2293,39 @@ FileReader[JO] val LOADING: Short
 FileReaderSync[JC] def readAsArrayBuffer(blob: Blob): ArrayBuffer
 FileReaderSync[JC] def readAsDataURL(blob: Blob): URL
 FileReaderSync[JC] def readAsText(blob: Blob, encoding: String?): String
+FileSystemCreateWritableOptions[JT] var keepExistingData: js.UndefOr[Boolean]
+FileSystemDirectoryHandle[JT] def getDirectoryHandle(name: String, options: js.UndefOr[FileSystemGetDirectoryOptions]?): js.Promise[FileSystemDirectoryHandle]
+FileSystemDirectoryHandle[JT] def getFileHandle(name: String, options: js.UndefOr[FileSystemGetFileOptions]?): js.Promise[FileSystemFileHandle]
+FileSystemDirectoryHandle[JT] def isSameEntry(fileSystemHandle: FileSystemHandle): js.Promise[Boolean]
+FileSystemDirectoryHandle[JT] val kind: FileSystemHandleKind
+FileSystemDirectoryHandle[JT] def name: String
+FileSystemDirectoryHandle[JT] def removeEntry(name: String, options: js.UndefOr[FileSystemRemoveOptions]?): js.Promise[Unit]
+FileSystemDirectoryHandle[JT] def resolve(possibleDescendant: FileSystemHandle): js.Promise[js.Array[String]]
+FileSystemFileHandle[JT] def createSyncAccessHandle(): js.Promise[FileSystemSyncAccessHandle]
+FileSystemFileHandle[JT] def createWritable(options: js.UndefOr[FileSystemCreateWritableOptions]?): js.Promise[FileSystemWritableFileStream]
+FileSystemFileHandle[JT] def getFile(): js.Promise[File]
+FileSystemGetDirectoryOptions[JT] var create: js.UndefOr[Boolean]
+FileSystemGetFileOptions[JT] var create: js.UndefOr[Boolean]
+FileSystemHandle[JT] def isSameEntry(fileSystemHandle: FileSystemHandle): js.Promise[Boolean]
+FileSystemHandle[JT] val kind: FileSystemHandleKind
+FileSystemHandle[JT] def name: String
+FileSystemHandleKind[JT]
+FileSystemHandleKind[SO] val directory: FileSystemHandleKind
+FileSystemHandleKind[SO] val file: FileSystemHandleKind
+FileSystemReadWriteOptions[JT] var at: Double
+FileSystemRemoveOptions[JT] var recursive: js.UndefOr[Boolean]
+FileSystemSyncAccessHandle[JT] def close(): Unit
+FileSystemSyncAccessHandle[JT] def flush(): Unit
+FileSystemSyncAccessHandle[JT] def getSize(): Double
+FileSystemSyncAccessHandle[JT] def read(buffer: BufferSource, options: js.UndefOr[FileSystemReadWriteOptions]?): Double
+FileSystemSyncAccessHandle[JT] def truncate(newSize: Double): Unit
+FileSystemSyncAccessHandle[JT] def write(buffer: BufferSource, options: js.UndefOr[FileSystemReadWriteOptions]?): Double
+FileSystemWritableFileStream[JT] def seek(position: Double): js.Promise[Unit]
+FileSystemWritableFileStream[JT] def truncate(size: Double): js.Promise[Unit]
+FileSystemWritableFileStream[JT] def write(data: Blob): js.Promise[Unit]
+FileSystemWritableFileStream[JT] def write(data: BufferSource): js.Promise[Unit]
+FileSystemWritableFileStream[JT] def write(data: String): js.Promise[Unit]
+FileSystemWritableFileStream[JT] def write(data: WriteParams): js.Promise[Unit]
 FocusEvent[JC] def bubbles: Boolean
 FocusEvent[JC] def cancelBubble: Boolean
 FocusEvent[JC] def cancelable: Boolean
@@ -26374,6 +26407,7 @@ StorageEventInit[JT] var scoped: js.UndefOr[Boolean]
 StorageEventInit[JT] var storageArea: js.UndefOr[Storage]
 StorageEventInit[JT] var url: js.UndefOr[String]
 StorageManager[JT] def estimate(): js.Promise[StorageEstimate]
+StorageManager[JT] def getDirectory(): js.Promise[FileSystemDirectoryHandle]
 StorageManager[JT] def persist(): js.Promise[Boolean]
 StorageManager[JT] def persisted(): js.Promise[Boolean]
 StyleMedia[JT] def matchMedium(mediaquery: String): Boolean
@@ -27574,6 +27608,14 @@ WorkerOptions[JT] var `type`: js.UndefOr[WorkerType]
 WorkerType[JT]
 WorkerType[SO] val classic: WorkerType
 WorkerType[SO] val module: WorkerType
+WriteCommandType[JT]
+WriteCommandType[SO] val seek: WriteCommandType
+WriteCommandType[SO] val truncate: WriteCommandType
+WriteCommandType[SO] val write: WriteCommandType
+WriteParams[JT] var data: js.UndefOr[BufferSource | Blob | String]
+WriteParams[JT] var position: js.UndefOr[Double]
+WriteParams[JT] var size: js.UndefOr[Double]
+WriteParams[JT] var `type`: WriteCommandType
 WriteableState[JT]
 WriteableState[SO] val closed: WriteableState
 WriteableState[SO] val closing: WriteableState
diff --git a/dom/src/main/scala-2/org/scalajs/dom/FileSystemHandleKind.scala b/dom/src/main/scala-2/org/scalajs/dom/FileSystemHandleKind.scala
new file mode 100644
index 000000000..c2fdfcf25
--- /dev/null
+++ b/dom/src/main/scala-2/org/scalajs/dom/FileSystemHandleKind.scala
@@ -0,0 +1,11 @@
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+@js.native
+sealed trait FileSystemHandleKind extends js.Any
+
+object FileSystemHandleKind {
+  val file: FileSystemHandleKind = "file".asInstanceOf[FileSystemHandleKind]
+  val directory: FileSystemHandleKind = "directory".asInstanceOf[FileSystemHandleKind]
+}
diff --git a/dom/src/main/scala-2/org/scalajs/dom/WriteCommandType.scala b/dom/src/main/scala-2/org/scalajs/dom/WriteCommandType.scala
new file mode 100644
index 000000000..a4500df59
--- /dev/null
+++ b/dom/src/main/scala-2/org/scalajs/dom/WriteCommandType.scala
@@ -0,0 +1,12 @@
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+@js.native
+sealed trait WriteCommandType extends js.Any
+
+object WriteCommandType {
+  val write: WriteCommandType = "write".asInstanceOf[WriteCommandType]
+  val seek: WriteCommandType = "seek".asInstanceOf[WriteCommandType]
+  val truncate: WriteCommandType = "truncate".asInstanceOf[WriteCommandType]
+}
diff --git a/dom/src/main/scala-3/org/scalajs/dom/FileSystemHandleKind.scala b/dom/src/main/scala-3/org/scalajs/dom/FileSystemHandleKind.scala
new file mode 100644
index 000000000..dc75561fc
--- /dev/null
+++ b/dom/src/main/scala-3/org/scalajs/dom/FileSystemHandleKind.scala
@@ -0,0 +1,10 @@
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+opaque type FileSystemHandleKind <: String = String
+
+object FileSystemHandleKind {
+  val file: FileSystemHandleKind = "file"
+  val directory: FileSystemHandleKind = "directory"
+}
diff --git a/dom/src/main/scala-3/org/scalajs/dom/WriteCommandType.scala b/dom/src/main/scala-3/org/scalajs/dom/WriteCommandType.scala
new file mode 100644
index 000000000..23433218b
--- /dev/null
+++ b/dom/src/main/scala-3/org/scalajs/dom/WriteCommandType.scala
@@ -0,0 +1,11 @@
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+opaque type WriteCommandType <: String = String
+
+object WriteCommandType {
+  val write: WriteCommandType = "write"
+  val seek: WriteCommandType = "seek"
+  val truncate: WriteCommandType = "truncate"
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemCreateWritableOptions.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemCreateWritableOptions.scala
new file mode 100644
index 000000000..8524c7741
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemCreateWritableOptions.scala
@@ -0,0 +1,17 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+trait FileSystemCreateWritableOptions extends js.Object {
+
+  /** A Boolean. Default false. When set to true if the file exists, the existing file is first copied to the temporary
+    * file. Otherwise the temporary file starts out empty.
+    */
+  var keepExistingData: js.UndefOr[Boolean] = js.undefined
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemDirectoryHandle.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemDirectoryHandle.scala
new file mode 100644
index 000000000..b25065a46
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemDirectoryHandle.scala
@@ -0,0 +1,67 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+/** The FileSystemDirectoryHandle interface of the File System API provides a handle to a file system directory. */
+@js.native
+trait FileSystemDirectoryHandle extends FileSystemHandle {
+
+  /** The getDirectoryHandle() method of the FileSystemDirectoryHandle interface returns a FileSystemDirectoryHandle for
+    * a subdirectory with the specified name within the directory handle on which the method is called.
+    *
+    * @param name
+    *   A string representing the FileSystemHandle.name of the file you wish to retrieve.
+    * @param options
+    *   An optional object containing options for the retrieved subdirectory.
+    *
+    * @return
+    *   A Promise which resolves with a FileSystemDirectoryHandle.
+    */
+  def getDirectoryHandle(name: String,
+      options: js.UndefOr[FileSystemGetDirectoryOptions] = js.native): js.Promise[FileSystemDirectoryHandle] = js.native
+
+  /** The getFileHandle() method of the FileSystemDirectoryHandle interface returns a FileSystemFileHandle for a file
+    * with the specified name, within the directory the method is called.
+    *
+    * @param name
+    *   A string representing the FileSystemHandle.name of the subdirectory you wish to retrieve.
+    * @param options
+    *   An object.
+    *
+    * @return
+    *   A Promise which resolves with a FileSystemFileHandle.
+    */
+  def getFileHandle(name: String,
+      options: js.UndefOr[FileSystemGetFileOptions] = js.native): js.Promise[FileSystemFileHandle] = js.native
+
+  /** The removeEntry() method of the FileSystemDirectoryHandle interface attempts to remove an entry if the directory
+    * handle contains a file or directory called the name specified.
+    *
+    * @param name
+    *   A string representing the FileSystemHandle.name of the entry you wish to remove.
+    * @param options
+    *   An optional object containing options
+    *
+    * @return
+    *   A Promise which resolves with undefined.
+    */
+  def removeEntry(name: String, options: js.UndefOr[FileSystemRemoveOptions] = js.native): js.Promise[Unit] = js.native
+
+  /** The resolve() method of the FileSystemDirectoryHandle interface returns an Array of directory names from the
+    * parent handle to the specified child entry, with the name of the child entry as the last array item.
+    *
+    * @param possibleDescendant
+    *   The FileSystemHandle from which to return the relative path.
+    *
+    * @return
+    *   A Promise which resolves with an Array of strings, or null if possibleDescendant is not a descendant of this
+    *   FileSystemDirectoryHandle.
+    */
+  def resolve(possibleDescendant: FileSystemHandle): js.Promise[js.Array[String]] = js.native
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemFileHandle.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemFileHandle.scala
new file mode 100644
index 000000000..c35d7bbd4
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemFileHandle.scala
@@ -0,0 +1,61 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+/** The FileSystemFileHandle interface of the File System API represents a handle to a file system entry. The interface
+  * is accessed through the window.showOpenFilePicker() method.
+  *
+  * Note that read and write operations depend on file-access permissions that do not persist after a page refresh if no
+  * other tabs for that origin remain open. The queryPermission method of the FileSystemHandle interface can be used to
+  * verify permission state before accessing a file.
+  */
+@js.native
+trait FileSystemFileHandle extends js.Object {
+
+  /** The getFile() method of the FileSystemFileHandle interface returns a Promise which resolves to a File object
+    * representing the state on disk of the entry represented by the handle.
+    *
+    * If the file on disk changes or is removed after this method is called, the returned File object will likely be no
+    * longer readable.
+    *
+    * @return
+    *   A Promise which resolves to a File object.
+    */
+  def getFile(): js.Promise[File] = js.native
+
+  /** The createSyncAccessHandle() method of the FileSystemFileHandle interface returns a Promise which resolves to a
+    * FileSystemSyncAccessHandle object that can be used to synchronously read from and write to a file. The synchronous
+    * nature of this method brings performance advantages, but it is only usable inside dedicated Web Workers for files
+    * within the origin private file system.
+    *
+    * Creating a FileSystemSyncAccessHandle takes an exclusive lock on the file associated with the file handle. This
+    * prevents the creation of further FileSystemSyncAccessHandles or FileSystemWritableFileStreams for the file until
+    * the existing access handle is closed.
+    *
+    * @return
+    *   A Promise which resolves to a FileSystemSyncAccessHandle object.
+    */
+  def createSyncAccessHandle(): js.Promise[FileSystemSyncAccessHandle] = js.native
+
+  /** The createWritable() method of the FileSystemFileHandle interface creates a FileSystemWritableFileStream that can
+    * be used to write to a file. The method returns a Promise which resolves to this created stream.
+    *
+    * Any changes made through the stream won't be reflected in the file represented by the file handle until the stream
+    * has been closed. This is typically implemented by writing data to a temporary file, and only replacing the file
+    * represented by file handle with the temporary file when the writable filestream is closed.
+    *
+    * @param options
+    *   An object.
+    *
+    * @return
+    *   A Promise which resolves to a FileSystemWritableFileStream object.
+    */
+  def createWritable(
+      options: js.UndefOr[FileSystemCreateWritableOptions] = js.native): js.Promise[FileSystemWritableFileStream] = js.native
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemGetDirectoryOptions.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemGetDirectoryOptions.scala
new file mode 100644
index 000000000..5271aaee3
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemGetDirectoryOptions.scala
@@ -0,0 +1,17 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+trait FileSystemGetDirectoryOptions extends js.Object {
+
+  /** A boolean value, which defaults to false. When set to true if the directory is not found, one with the specified
+    * name will be created and returned.
+    */
+  var create: js.UndefOr[Boolean] = js.undefined
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemGetFileOptions.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemGetFileOptions.scala
new file mode 100644
index 000000000..2c39f3adb
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemGetFileOptions.scala
@@ -0,0 +1,17 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+trait FileSystemGetFileOptions extends js.Object {
+
+  /** A Boolean. Default false. When set to true if the file is not found, one with the specified name will be created
+    * and returned.
+    */
+  var create: js.UndefOr[Boolean] = js.undefined
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemHandle.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemHandle.scala
new file mode 100644
index 000000000..a6ad25c59
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemHandle.scala
@@ -0,0 +1,34 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+/** The FileSystemHandle interface of the File System API is an object which represents a file or directory entry.
+  * Multiple handles can represent the same entry. For the most part you do not work with FileSystemHandle directly but
+  * rather its child interfaces FileSystemFileHandle and FileSystemDirectoryHandle.
+  */
+@js.native
+trait FileSystemHandle extends js.Object {
+
+  /** Returns the type of entry. This is 'file' if the associated entry is a file or 'directory'. */
+  val kind: FileSystemHandleKind = js.native
+
+  /** Returns the name of the associated entry. */
+  def name: String = js.native
+
+  /** The isSameEntry() method of the FileSystemHandle interface compares two handles to see if the associated entries
+    * (either a file or directory) match.
+    *
+    * @param fileSystemHandle
+    *   The FileSystemHandle to match against the handle on which the method is invoked.
+    *
+    * @return
+    *   A Promise that fulfills with a Boolean.
+    */
+  def isSameEntry(fileSystemHandle: FileSystemHandle): js.Promise[Boolean] = js.native
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemReadWriteOptions.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemReadWriteOptions.scala
new file mode 100644
index 000000000..59898b85a
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemReadWriteOptions.scala
@@ -0,0 +1,17 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+trait FileSystemReadWriteOptions extends js.Object {
+
+  /** A number representing the offset in bytes from the start of the file that the file should be read from or written
+    * at.
+    */
+  var at: Double
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemRemoveOptions.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemRemoveOptions.scala
new file mode 100644
index 000000000..9dff814f8
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemRemoveOptions.scala
@@ -0,0 +1,15 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+trait FileSystemRemoveOptions extends js.Object {
+
+  /** A boolean value, which defaults to false. When set to true entries will be removed recursively. */
+  var recursive: js.UndefOr[Boolean] = js.undefined
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemSyncAccessHandle.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemSyncAccessHandle.scala
new file mode 100644
index 000000000..6c3435a68
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemSyncAccessHandle.scala
@@ -0,0 +1,87 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+/** The FileSystemSyncAccessHandle interface of the File System API represents a synchronous handle to a file system
+  * entry.
+  *
+  * This class is only accessible inside dedicated Web Workers (so that its methods do not block execution on the main
+  * thread) for files within the origin private file system, which is not visible to end-users.
+  *
+  * As a result, its methods are not subject to the same security checks as methods running on files within the
+  * user-visible file system, and so are much more performant. This makes them suitable for significant, large-scale
+  * file updates such as SQLite database modifications.
+  *
+  * The interface is accessed through the FileSystemFileHandle.createSyncAccessHandle() method.
+  */
+@js.native
+trait FileSystemSyncAccessHandle extends js.Object {
+
+  /** The close() method of the FileSystemSyncAccessHandle interface closes an open synchronous file handle, disabling
+    * any further operations on it and releasing the exclusive lock previously put on the file associated with the file
+    * handle.
+    */
+  def close(): Unit = js.native
+
+  /** The flush() method of the FileSystemSyncAccessHandle interface persists any changes made to the file associated
+    * with the handle via the write() method to disk.
+    *
+    * Bear in mind that you only need to call this method if you need the changes committed to disk at a specific time,
+    * otherwise you can leave the underlying operating system to handle this when it sees fit, which should be OK in
+    * most cases.
+    */
+  def flush(): Unit = js.native
+
+  /** The getSize() method of the FileSystemSyncAccessHandle interface returns the size of the file associated with the
+    * handle in bytes.
+    */
+  def getSize(): Double = js.native
+
+  /** The read() method of the FileSystemSyncAccessHandle interface reads the content of the file associated with the
+    * handle into a specified buffer, optionally at a given offset.
+    *
+    * @param buffer
+    *   An ArrayBuffer or ArrayBufferView (such as a DataView) representing the buffer that the file content should be
+    *   read into. Note that you cannot directly manipulate the contents of an ArrayBuffer. Instead, you create one of
+    *   the typed array objects like an Int8Array or a DataView object which represents the buffer in a specific format,
+    *   and use that to read and write the contents of the buffer.
+    * @param options
+    *   An options object.
+    *
+    * @return
+    *   A number representing the number of bytes read from the file.
+    */
+  def read(buffer: BufferSource, options: js.UndefOr[FileSystemReadWriteOptions] = js.native): Double = js.native
+
+  /** The truncate() method of the FileSystemSyncAccessHandle interface resizes the file associated with the handle to a
+    * specified number of bytes.
+    *
+    * @param newSize
+    *   The number of bytes to resize the file to.
+    */
+  def truncate(newSize: Double): Unit = js.native
+
+  /** The write() method of the FileSystemSyncAccessHandle interface writes the content of a specified buffer to the
+    * file associated with the handle, optionally at a given offset.
+    *
+    * Files within the origin private file system are not visible to end-users, therefore are not subject to the same
+    * security checks as methods running on files within the user-visible file system. As a result, writes performed
+    * using FileSystemSyncAccessHandle.write() are much more performant. This makes them suitable for significant,
+    * large-scale file updates such as SQLite database modifications.
+    *
+    * @param buffer
+    *   An ArrayBuffer or ArrayBufferView (such as a DataView) representing the buffer to be written to the file.
+    * @param options
+    *   An options object.
+    *
+    * @return
+    *   A number representing the number of bytes written to the file.
+    */
+  def write(buffer: BufferSource, options: js.UndefOr[FileSystemReadWriteOptions] = js.native): Double = js.native
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/FileSystemWritableFileStream.scala b/dom/src/main/scala/org/scalajs/dom/FileSystemWritableFileStream.scala
new file mode 100644
index 000000000..7796b79df
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/FileSystemWritableFileStream.scala
@@ -0,0 +1,71 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+/** The FileSystemWritableFileStream interface of the File System API is a WritableStream object with additional
+  * convenience methods, which operates on a single file on disk. The interface is accessed through the
+  * FileSystemFileHandle.createWritable() method.
+  */
+@js.native
+trait FileSystemWritableFileStream extends js.Object {
+
+  /** The write() method of the FileSystemWritableFileStream interface writes content into the file the method is called
+    * on, at the current file cursor offset.
+    *
+    * No changes are written to the actual file on disk until the stream has been closed. Changes are typically written
+    * to a temporary file instead. This method can also be used to seek to a byte point within the stream and truncate
+    * to modify the total bytes the file contains.
+    *
+    * @param data
+    *   Can be one of the following:
+    *   - The file data to write, in the form of an ArrayBuffer, TypedArray, DataView, Blob, or string.
+    *   - A WriteParams object.
+    *
+    * @return
+    *   A Promise that returns undefined.
+    */
+  def write(data: BufferSource): js.Promise[Unit] = js.native
+
+  def write(data: Blob): js.Promise[Unit] = js.native
+
+  def write(data: String): js.Promise[Unit] = js.native
+
+  def write(data: WriteParams): js.Promise[Unit] = js.native
+
+  /** The seek() method of the FileSystemWritableFileStream interface updates the current file cursor offset to the
+    * position (in bytes) specified when calling the method.
+    *
+    * @param position
+    *   A number specifying the byte position from the beginning of the file.
+    *
+    * @return
+    *   A Promise that returns undefined.
+    */
+  def seek(position: Double): js.Promise[Unit] = js.native
+
+  /** The truncate() method of the FileSystemWritableFileStream interface resizes the file associated with the stream to
+    * the specified size in bytes.
+    *
+    * If the size specified is larger than the current file size the file is padded with 0x00 bytes.
+    *
+    * The file cursor is also updated when truncate() is called. If the offset is smaller than the size, it remains
+    * unchanged. If the offset is larger than size, the offset is set to that size. This ensures that subsequent writes
+    * do not error.
+    *
+    * No changes are written to the actual file on disk until the stream has been closed. Changes are typically written
+    * to a temporary file instead.
+    *
+    * @param size
+    *   A number specifying the number of bytes to resize the stream to.
+    *
+    * @return
+    *   A Promise that returns undefined.
+    */
+  def truncate(size: Double): js.Promise[Unit] = js.native
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/StorageManager.scala b/dom/src/main/scala/org/scalajs/dom/StorageManager.scala
index 2d1f2e839..e6e601238 100644
--- a/dom/src/main/scala/org/scalajs/dom/StorageManager.scala
+++ b/dom/src/main/scala/org/scalajs/dom/StorageManager.scala
@@ -1,3 +1,9 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
 package org.scalajs.dom
 
 import scala.scalajs.js
@@ -7,4 +13,10 @@ trait StorageManager extends js.Object {
   def persisted(): js.Promise[Boolean] = js.native
   def persist(): js.Promise[Boolean] = js.native
   def estimate(): js.Promise[StorageEstimate] = js.native
+
+  /** The getDirectory() method of the StorageManager interface is used to obtain a reference to a
+    * FileSystemDirectoryHandle object allowing access to a directory and its contents, stored in the origin private
+    * file system (OPFS).
+    */
+  def getDirectory(): js.Promise[FileSystemDirectoryHandle] = js.native
 }
diff --git a/dom/src/main/scala/org/scalajs/dom/WriteParams.scala b/dom/src/main/scala/org/scalajs/dom/WriteParams.scala
new file mode 100644
index 000000000..e1b3e1856
--- /dev/null
+++ b/dom/src/main/scala/org/scalajs/dom/WriteParams.scala
@@ -0,0 +1,31 @@
+/** All documentation for facades is thanks to Mozilla Contributors at https://developer.mozilla.org/en-US/docs/Web/API
+  * and available under the Creative Commons Attribution-ShareAlike v2.5 or later.
+  * http://creativecommons.org/licenses/by-sa/2.5/
+  *
+  * Everything else is under the MIT License http://opensource.org/licenses/MIT
+  */
+package org.scalajs.dom
+
+import scala.scalajs.js
+import scala.scalajs.js.|
+
+trait WriteParams extends js.Object {
+
+  /** A string that is one of "write", "seek", or "truncate". */
+  var `type`: WriteCommandType
+
+  /** The file data to write. Can be an ArrayBuffer, TypedArray, DataView, Blob, or string. This property is required if
+    * type is set to "write".
+    */
+  var data: js.UndefOr[BufferSource | Blob | String] = js.undefined
+
+  /** The byte position the current file cursor should move to if type "seek" is used. Can also be set if type is
+    * "write", in which case the write will start at the specified position.
+    */
+  var position: js.UndefOr[Double] = js.undefined
+
+  /** A number representing the number of bytes the stream should contain. This property is required if type is set to
+    * "truncate".
+    */
+  var size: js.UndefOr[Double] = js.undefined
+}