diff --git a/shell/app/uv_task_runner.cc b/shell/app/uv_task_runner.cc
index 905c64549fa..f98ddce8737 100644
--- a/shell/app/uv_task_runner.cc
+++ b/shell/app/uv_task_runner.cc
@@ -4,9 +4,9 @@
 
 #include <utility>
 
-#include "shell/app/uv_task_runner.h"
-
+#include "base/location.h"
 #include "base/stl_util.h"
+#include "shell/app/uv_task_runner.h"
 
 namespace electron {
 
diff --git a/shell/app/uv_task_runner.h b/shell/app/uv_task_runner.h
index cd7b55005c6..621dd08965a 100644
--- a/shell/app/uv_task_runner.h
+++ b/shell/app/uv_task_runner.h
@@ -8,10 +8,13 @@
 #include <map>
 
 #include "base/callback.h"
-#include "base/location.h"
 #include "base/single_thread_task_runner.h"
 #include "uv.h"  // NOLINT(build/include_directory)
 
+namespace base {
+class Location;
+}
+
 namespace electron {
 
 // TaskRunner implementation that posts tasks into libuv's default loop.
diff --git a/shell/browser/api/electron_api_browser_view.h b/shell/browser/api/electron_api_browser_view.h
index 9ceba81c01b..1f4e9418bed 100644
--- a/shell/browser/api/electron_api_browser_view.h
+++ b/shell/browser/api/electron_api_browser_view.h
@@ -29,8 +29,6 @@ class Dictionary;
 
 namespace electron {
 
-class NativeBrowserView;
-
 namespace api {
 
 class WebContents;
diff --git a/shell/browser/api/electron_api_cookies.h b/shell/browser/api/electron_api_cookies.h
index 090399c1788..087d723e445 100644
--- a/shell/browser/api/electron_api_cookies.h
+++ b/shell/browser/api/electron_api_cookies.h
@@ -23,10 +23,6 @@ namespace gin_helper {
 class Dictionary;
 }
 
-namespace net {
-class URLRequestContextGetter;
-}
-
 namespace electron {
 
 class ElectronBrowserContext;
diff --git a/shell/browser/api/electron_api_download_item.cc b/shell/browser/api/electron_api_download_item.cc
index e285e962a1a..6943d8effcc 100644
--- a/shell/browser/api/electron_api_download_item.cc
+++ b/shell/browser/api/electron_api_download_item.cc
@@ -16,6 +16,7 @@
 #include "shell/common/gin_helper/dictionary.h"
 #include "shell/common/gin_helper/object_template_builder.h"
 #include "shell/common/node_includes.h"
+#include "url/gurl.h"
 
 namespace gin {
 
diff --git a/shell/browser/api/electron_api_download_item.h b/shell/browser/api/electron_api_download_item.h
index ce12dfbdbc6..bb67279ac9a 100644
--- a/shell/browser/api/electron_api_download_item.h
+++ b/shell/browser/api/electron_api_download_item.h
@@ -15,7 +15,8 @@
 #include "shell/browser/event_emitter_mixin.h"
 #include "shell/browser/ui/file_dialog.h"
 #include "shell/common/gin_helper/pinnable.h"
-#include "url/gurl.h"
+
+class GURL;
 
 namespace electron {
 
diff --git a/shell/browser/api/electron_api_protocol.cc b/shell/browser/api/electron_api_protocol.cc
index 164615ffcd2..70bbcd769a7 100644
--- a/shell/browser/api/electron_api_protocol.cc
+++ b/shell/browser/api/electron_api_protocol.cc
@@ -19,6 +19,7 @@
 #include "shell/common/gin_helper/dictionary.h"
 #include "shell/common/gin_helper/object_template_builder.h"
 #include "shell/common/gin_helper/promise.h"
+#include "shell/common/node_includes.h"
 #include "shell/common/options_switches.h"
 #include "shell/common/process_util.h"
 #include "url/url_util.h"
diff --git a/shell/browser/api/electron_api_system_preferences_mac.mm b/shell/browser/api/electron_api_system_preferences_mac.mm
index 9281837fc07..224cd7966db 100644
--- a/shell/browser/api/electron_api_system_preferences_mac.mm
+++ b/shell/browser/api/electron_api_system_preferences_mac.mm
@@ -29,6 +29,7 @@
 #include "shell/common/color_util.h"
 #include "shell/common/gin_converters/gurl_converter.h"
 #include "shell/common/gin_converters/value_converter.h"
+#include "shell/common/node_includes.h"
 #include "shell/common/process_util.h"
 #include "skia/ext/skia_utils_mac.h"
 #include "ui/native_theme/native_theme.h"
diff --git a/shell/browser/api/electron_api_tray.h b/shell/browser/api/electron_api_tray.h
index f3ce64ae5bd..bf6f9100e3c 100644
--- a/shell/browser/api/electron_api_tray.h
+++ b/shell/browser/api/electron_api_tray.h
@@ -33,7 +33,6 @@ namespace electron {
 namespace api {
 
 class Menu;
-class NativeImage;
 
 class Tray : public gin::Wrappable<Tray>,
              public gin_helper::EventEmitterMixin<Tray>,
diff --git a/shell/browser/api/electron_api_web_contents_view.h b/shell/browser/api/electron_api_web_contents_view.h
index 26e2c602562..86c65c22c2e 100644
--- a/shell/browser/api/electron_api_web_contents_view.h
+++ b/shell/browser/api/electron_api_web_contents_view.h
@@ -14,8 +14,6 @@ class Dictionary;
 
 namespace electron {
 
-class InspectableWebContents;
-
 namespace api {
 
 class WebContents;
diff --git a/shell/browser/api/electron_api_web_frame_main.h b/shell/browser/api/electron_api_web_frame_main.h
index 37be0a2eebc..37c3ba3d707 100644
--- a/shell/browser/api/electron_api_web_frame_main.h
+++ b/shell/browser/api/electron_api_web_frame_main.h
@@ -27,10 +27,6 @@ namespace gin {
 class Arguments;
 }
 
-namespace gin_helper {
-class Dictionary;
-}
-
 namespace electron {
 
 namespace api {
diff --git a/shell/browser/api/event.h b/shell/browser/api/event.h
index d10e9b47a3a..5cc7c47750b 100644
--- a/shell/browser/api/event.h
+++ b/shell/browser/api/event.h
@@ -9,10 +9,6 @@
 #include "gin/handle.h"
 #include "gin/wrappable.h"
 
-namespace IPC {
-class Message;
-}
-
 namespace gin_helper {
 
 class Event : public gin::Wrappable<Event> {
diff --git a/shell/browser/api/frame_subscriber.h b/shell/browser/api/frame_subscriber.h
index 486cee45459..031c3a591b9 100644
--- a/shell/browser/api/frame_subscriber.h
+++ b/shell/browser/api/frame_subscriber.h
@@ -18,7 +18,8 @@
 
 namespace gfx {
 class Image;
-}
+class Rect;
+}  // namespace gfx
 
 namespace electron {
 
diff --git a/shell/browser/bluetooth/electron_bluetooth_delegate.h b/shell/browser/bluetooth/electron_bluetooth_delegate.h
index 883eef11000..96a6103a16f 100644
--- a/shell/browser/bluetooth/electron_bluetooth_delegate.h
+++ b/shell/browser/bluetooth/electron_bluetooth_delegate.h
@@ -17,11 +17,11 @@
 
 namespace blink {
 class WebBluetoothDeviceId;
-}  // namespace blink
+}
 
 namespace content {
 class RenderFrameHost;
-}  // namespace content
+}
 
 namespace device {
 class BluetoothDevice;
diff --git a/shell/browser/browser.h b/shell/browser/browser.h
index 682f5b1b771..3b57855255c 100755
--- a/shell/browser/browser.h
+++ b/shell/browser/browser.h
@@ -33,10 +33,6 @@ namespace base {
 class FilePath;
 }
 
-namespace gfx {
-class Image;
-}
-
 namespace gin_helper {
 class Arguments;
 }
diff --git a/shell/browser/browser_process_impl.h b/shell/browser/browser_process_impl.h
index ac42e84d45f..1f9ad3b97d5 100644
--- a/shell/browser/browser_process_impl.h
+++ b/shell/browser/browser_process_impl.h
@@ -26,7 +26,7 @@ namespace printing {
 class PrintJobManager;
 }
 
-// Empty definition for std::unique_ptr
+// Empty definition for std::unique_ptr, rather than a forward declaration
 class BackgroundModeManager {};
 
 // NOT THREAD SAFE, call only from the main thread.
diff --git a/shell/browser/electron_browser_context.h b/shell/browser/electron_browser_context.h
index 87052a37c77..4cb737bcdff 100644
--- a/shell/browser/electron_browser_context.h
+++ b/shell/browser/electron_browser_context.h
@@ -18,7 +18,6 @@
 #include "services/network/public/mojom/url_loader_factory.mojom.h"
 #include "shell/browser/media/media_device_id_salt.h"
 
-class PrefRegistrySimple;
 class PrefService;
 class ValueMapPrefStore;
 
@@ -43,7 +42,6 @@ class ElectronDownloadManagerDelegate;
 class ElectronPermissionManager;
 class CookieChangeNotifier;
 class ResolveProxyHelper;
-class SpecialStoragePolicy;
 class WebViewManager;
 class ProtocolRegistry;
 
diff --git a/shell/browser/electron_browser_handler_impl.h b/shell/browser/electron_browser_handler_impl.h
index 06a5c0a6605..fc2eab200d9 100644
--- a/shell/browser/electron_browser_handler_impl.h
+++ b/shell/browser/electron_browser_handler_impl.h
@@ -16,7 +16,7 @@
 
 namespace content {
 class RenderFrameHost;
-}  // namespace content
+}
 
 namespace electron {
 class ElectronBrowserHandlerImpl : public mojom::ElectronBrowser,
diff --git a/shell/browser/electron_browser_main_parts.cc b/shell/browser/electron_browser_main_parts.cc
index be430e87836..5647a75fb64 100644
--- a/shell/browser/electron_browser_main_parts.cc
+++ b/shell/browser/electron_browser_main_parts.cc
@@ -11,6 +11,7 @@
 #include "base/base_switches.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
+#include "base/metrics/field_trial.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
diff --git a/shell/browser/electron_browser_main_parts.h b/shell/browser/electron_browser_main_parts.h
index 1da21556fbd..8ad163a3ead 100644
--- a/shell/browser/electron_browser_main_parts.h
+++ b/shell/browser/electron_browser_main_parts.h
@@ -9,7 +9,6 @@
 #include <string>
 
 #include "base/callback.h"
-#include "base/metrics/field_trial.h"
 #include "base/timer/timer.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/browser_main_parts.h"
@@ -20,9 +19,13 @@
 #include "third_party/abseil-cpp/absl/types/optional.h"
 #include "ui/views/layout/layout_provider.h"
 
-class BrowserProcess;
+class BrowserProcessImpl;
 class IconManager;
 
+namespace base {
+class FieldTrialList;
+}
+
 #if defined(USE_AURA)
 namespace wm {
 class WMState;
@@ -41,17 +44,15 @@ class GtkUiPlatform;
 
 namespace device {
 class GeolocationManager;
-}  // namespace device
+}
 
 namespace electron {
 
-class ElectronBrowserContext;
 class Browser;
 class ElectronBindings;
 class JavascriptEnvironment;
 class NodeBindings;
 class NodeEnvironment;
-class BridgeTaskRunner;
 
 #if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
 class ElectronExtensionsClient;
diff --git a/shell/browser/electron_browser_main_parts_posix.cc b/shell/browser/electron_browser_main_parts_posix.cc
index a8a9cf8db19..1618cbdcbdb 100644
--- a/shell/browser/electron_browser_main_parts_posix.cc
+++ b/shell/browser/electron_browser_main_parts_posix.cc
@@ -13,6 +13,7 @@
 #include <sys/resource.h>
 #include <unistd.h>
 
+#include "base/debug/leak_annotations.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/task/post_task.h"
 #include "content/public/browser/browser_task_traits.h"
diff --git a/shell/browser/electron_javascript_dialog_manager.h b/shell/browser/electron_javascript_dialog_manager.h
index 4c5135c9eda..e36778696ba 100644
--- a/shell/browser/electron_javascript_dialog_manager.h
+++ b/shell/browser/electron_javascript_dialog_manager.h
@@ -10,12 +10,12 @@
 
 #include "content/public/browser/javascript_dialog_manager.h"
 
-namespace electron {
-
-namespace api {
+namespace content {
 class WebContents;
 }
 
+namespace electron {
+
 class ElectronJavaScriptDialogManager
     : public content::JavaScriptDialogManager {
  public:
diff --git a/shell/browser/electron_permission_manager.cc b/shell/browser/electron_permission_manager.cc
index c10f37b029b..78dd47fa2f4 100644
--- a/shell/browser/electron_permission_manager.cc
+++ b/shell/browser/electron_permission_manager.cc
@@ -8,6 +8,7 @@
 #include <utility>
 #include <vector>
 
+#include "base/values.h"
 #include "content/public/browser/child_process_security_policy.h"
 #include "content/public/browser/global_routing_id.h"
 #include "content/public/browser/permission_controller.h"
diff --git a/shell/browser/electron_permission_manager.h b/shell/browser/electron_permission_manager.h
index f1bab317cb9..c4d8b5bd7e5 100644
--- a/shell/browser/electron_permission_manager.h
+++ b/shell/browser/electron_permission_manager.h
@@ -10,9 +10,13 @@
 
 #include "base/callback.h"
 #include "base/containers/id_map.h"
-#include "base/values.h"
 #include "content/public/browser/permission_controller_delegate.h"
 
+namespace base {
+class DictionaryValue;
+class Value;
+}  // namespace base
+
 namespace content {
 class WebContents;
 }
diff --git a/shell/browser/electron_quota_permission_context.cc b/shell/browser/electron_quota_permission_context.cc
index 7c0f98d2195..cdb79b53311 100644
--- a/shell/browser/electron_quota_permission_context.cc
+++ b/shell/browser/electron_quota_permission_context.cc
@@ -6,6 +6,8 @@
 
 #include <utility>
 
+#include "content/public/common/storage_quota_params.h"
+
 namespace electron {
 
 ElectronQuotaPermissionContext::ElectronQuotaPermissionContext() = default;
diff --git a/shell/browser/electron_quota_permission_context.h b/shell/browser/electron_quota_permission_context.h
index 5312b968d40..7418718cc87 100644
--- a/shell/browser/electron_quota_permission_context.h
+++ b/shell/browser/electron_quota_permission_context.h
@@ -6,7 +6,10 @@
 #define SHELL_BROWSER_ELECTRON_QUOTA_PERMISSION_CONTEXT_H_
 
 #include "content/public/browser/quota_permission_context.h"
-#include "content/public/common/storage_quota_params.h"
+
+namespace content {
+struct StorageQuotaParams;
+}
 
 namespace electron {
 
diff --git a/shell/browser/electron_web_ui_controller_factory.cc b/shell/browser/electron_web_ui_controller_factory.cc
index c6e159ce7c6..b28807c9dd2 100644
--- a/shell/browser/electron_web_ui_controller_factory.cc
+++ b/shell/browser/electron_web_ui_controller_factory.cc
@@ -6,6 +6,7 @@
 
 #include "chrome/common/webui_url_constants.h"
 #include "content/public/browser/web_contents.h"
+#include "content/public/browser/web_ui_controller.h"
 #include "electron/buildflags/buildflags.h"
 #include "shell/browser/ui/devtools_ui.h"
 #include "shell/browser/ui/webui/accessibility_ui.h"
diff --git a/shell/browser/electron_web_ui_controller_factory.h b/shell/browser/electron_web_ui_controller_factory.h
index d001f933424..ea1e245c720 100644
--- a/shell/browser/electron_web_ui_controller_factory.h
+++ b/shell/browser/electron_web_ui_controller_factory.h
@@ -9,9 +9,12 @@
 
 #include "base/macros.h"
 #include "base/memory/singleton.h"
-#include "content/public/browser/web_ui_controller.h"
 #include "content/public/browser/web_ui_controller_factory.h"
 
+namespace content {
+class WebUIController;
+}
+
 namespace electron {
 
 class ElectronWebUIControllerFactory : public content::WebUIControllerFactory {
diff --git a/shell/browser/extensions/api/runtime/electron_runtime_api_delegate.h b/shell/browser/extensions/api/runtime/electron_runtime_api_delegate.h
index 1ddc45e317d..bce649e6f93 100644
--- a/shell/browser/extensions/api/runtime/electron_runtime_api_delegate.h
+++ b/shell/browser/extensions/api/runtime/electron_runtime_api_delegate.h
@@ -12,7 +12,7 @@
 
 namespace content {
 class BrowserContext;
-}  // namespace content
+}
 
 namespace extensions {
 
diff --git a/shell/browser/extensions/api/tabs/tabs_api.cc b/shell/browser/extensions/api/tabs/tabs_api.cc
index 60838cd8891..4c56bcd33e7 100644
--- a/shell/browser/extensions/api/tabs/tabs_api.cc
+++ b/shell/browser/extensions/api/tabs/tabs_api.cc
@@ -16,6 +16,7 @@
 #include "shell/browser/web_contents_zoom_controller.h"
 #include "shell/common/extensions/api/tabs.h"
 #include "third_party/blink/public/common/page/page_zoom.h"
+#include "url/gurl.h"
 
 using electron::WebContentsZoomController;
 
diff --git a/shell/browser/extensions/api/tabs/tabs_api.h b/shell/browser/extensions/api/tabs/tabs_api.h
index b8a424362b4..fe3261fcfdd 100644
--- a/shell/browser/extensions/api/tabs/tabs_api.h
+++ b/shell/browser/extensions/api/tabs/tabs_api.h
@@ -10,7 +10,8 @@
 #include "extensions/browser/api/execute_code_function.h"
 #include "extensions/browser/extension_function.h"
 #include "extensions/common/extension_resource.h"
-#include "url/gurl.h"
+
+class GURL;
 
 namespace extensions {
 
diff --git a/shell/browser/extensions/electron_extension_loader.h b/shell/browser/extensions/electron_extension_loader.h
index 49c6f630983..cc3ddfcacf0 100644
--- a/shell/browser/extensions/electron_extension_loader.h
+++ b/shell/browser/extensions/electron_extension_loader.h
@@ -17,11 +17,11 @@
 
 namespace base {
 class FilePath;
-}  // namespace base
+}
 
 namespace content {
 class BrowserContext;
-}  // namespace content
+}
 
 namespace extensions {
 
diff --git a/shell/browser/native_window.cc b/shell/browser/native_window.cc
index 435b0078356..a39054cf421 100644
--- a/shell/browser/native_window.cc
+++ b/shell/browser/native_window.cc
@@ -10,6 +10,7 @@
 
 #include "base/memory/ptr_util.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/values.h"
 #include "shell/browser/browser.h"
 #include "shell/browser/window_list.h"
 #include "shell/common/color_util.h"
diff --git a/shell/browser/native_window.h b/shell/browser/native_window.h
index 1241d70f8ce..cab20e57bb3 100644
--- a/shell/browser/native_window.h
+++ b/shell/browser/native_window.h
@@ -13,7 +13,6 @@
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "base/supports_user_data.h"
-#include "base/values.h"
 #include "content/public/browser/desktop_media_id.h"
 #include "content/public/browser/web_contents_user_data.h"
 #include "extensions/browser/app_window/size_constraints.h"
@@ -21,7 +20,9 @@
 #include "third_party/abseil-cpp/absl/types/optional.h"
 #include "ui/views/widget/widget_delegate.h"
 
-class SkRegion;
+namespace base {
+class DictionaryValue;
+}
 
 namespace content {
 struct NativeWebKeyboardEvent;
@@ -31,7 +32,6 @@ namespace gfx {
 class Image;
 class Point;
 class Rect;
-class RectF;
 enum class ResizeEdge;
 class Size;
 }  // namespace gfx
diff --git a/shell/browser/native_window_mac.mm b/shell/browser/native_window_mac.mm
index f89b25bd9de..29cba7eaa09 100644
--- a/shell/browser/native_window_mac.mm
+++ b/shell/browser/native_window_mac.mm
@@ -38,6 +38,7 @@
 #include "shell/browser/window_list.h"
 #include "shell/common/gin_converters/gfx_converter.h"
 #include "shell/common/gin_helper/dictionary.h"
+#include "shell/common/node_includes.h"
 #include "shell/common/options_switches.h"
 #include "shell/common/process_util.h"
 #include "skia/ext/skia_utils_mac.h"
diff --git a/shell/browser/native_window_observer.h b/shell/browser/native_window_observer.h
index caa38e4ffa1..ce7f337c3c1 100644
--- a/shell/browser/native_window_observer.h
+++ b/shell/browser/native_window_observer.h
@@ -10,12 +10,13 @@
 #include "base/observer_list_types.h"
 #include "base/values.h"
 #include "ui/base/window_open_disposition.h"
-#include "url/gurl.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
 
+class GURL;
+
 namespace gfx {
 class Rect;
 enum class ResizeEdge;
diff --git a/shell/browser/net/network_context_service_factory.h b/shell/browser/net/network_context_service_factory.h
index 187aa25b0c8..8d24457ed5d 100644
--- a/shell/browser/net/network_context_service_factory.h
+++ b/shell/browser/net/network_context_service_factory.h
@@ -10,7 +10,7 @@
 
 class KeyedService;
 
-namespace contenet {
+namespace content {
 class BrowserContext;
 }
 
diff --git a/shell/browser/net/system_network_context_manager.h b/shell/browser/net/system_network_context_manager.h
index 588977cac07..6d65e04ca16 100644
--- a/shell/browser/net/system_network_context_manager.h
+++ b/shell/browser/net/system_network_context_manager.h
@@ -16,10 +16,6 @@
 #include "services/network/public/mojom/url_loader_factory.mojom.h"
 #include "third_party/abseil-cpp/absl/types/optional.h"
 
-namespace net_log {
-class NetExportFileWriter;
-}
-
 namespace electron {
 network::mojom::HttpAuthDynamicParamsPtr CreateHttpAuthDynamicParams();
 }
diff --git a/shell/browser/osr/osr_render_widget_host_view.h b/shell/browser/osr/osr_render_widget_host_view.h
index 51eaa290aca..477c3047870 100644
--- a/shell/browser/osr/osr_render_widget_host_view.h
+++ b/shell/browser/osr/osr_render_widget_host_view.h
@@ -43,7 +43,7 @@
 
 namespace content {
 class CursorManager;
-}  // namespace content
+}
 
 namespace electron {
 
diff --git a/shell/browser/pref_store_delegate.cc b/shell/browser/pref_store_delegate.cc
index 34141f0dca9..57740077bc4 100644
--- a/shell/browser/pref_store_delegate.cc
+++ b/shell/browser/pref_store_delegate.cc
@@ -6,6 +6,8 @@
 
 #include <utility>
 
+#include "components/prefs/persistent_pref_store.h"
+#include "components/prefs/pref_store.h"
 #include "components/prefs/value_map_pref_store.h"
 #include "shell/browser/electron_browser_context.h"
 
diff --git a/shell/browser/pref_store_delegate.h b/shell/browser/pref_store_delegate.h
index 548696bcb77..22ab480d43d 100644
--- a/shell/browser/pref_store_delegate.h
+++ b/shell/browser/pref_store_delegate.h
@@ -9,9 +9,13 @@
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
-#include "components/prefs/persistent_pref_store.h"
 #include "components/prefs/pref_value_store.h"
 
+class PersistentPrefStore;
+class PrefNotifier;
+class PrefRegistry;
+class PrefStore;
+
 namespace electron {
 
 class ElectronBrowserContext;
diff --git a/shell/browser/protocol_registry.h b/shell/browser/protocol_registry.h
index 3ea7e12744b..f5a07863ec8 100644
--- a/shell/browser/protocol_registry.h
+++ b/shell/browser/protocol_registry.h
@@ -12,7 +12,7 @@
 
 namespace content {
 class BrowserContext;
-}  // namespace content
+}
 
 namespace electron {
 
diff --git a/shell/browser/serial/serial_chooser_controller.h b/shell/browser/serial/serial_chooser_controller.h
index 85c37590825..545e5d8bafb 100644
--- a/shell/browser/serial/serial_chooser_controller.h
+++ b/shell/browser/serial/serial_chooser_controller.h
@@ -21,7 +21,7 @@
 
 namespace content {
 class RenderFrameHost;
-}  // namespace content
+}
 
 namespace electron {
 
diff --git a/shell/browser/session_preferences.cc b/shell/browser/session_preferences.cc
index 2cdce00d2fa..438940a2bfc 100644
--- a/shell/browser/session_preferences.cc
+++ b/shell/browser/session_preferences.cc
@@ -6,6 +6,7 @@
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
+#include "content/public/browser/browser_context.h"
 
 namespace electron {
 
diff --git a/shell/browser/session_preferences.h b/shell/browser/session_preferences.h
index 958f4789d30..e02d92ee47e 100644
--- a/shell/browser/session_preferences.h
+++ b/shell/browser/session_preferences.h
@@ -9,7 +9,10 @@
 
 #include "base/files/file_path.h"
 #include "base/supports_user_data.h"
-#include "content/public/browser/browser_context.h"
+
+namespace content {
+class BrowserContext;
+}
 
 namespace electron {
 
diff --git a/shell/browser/ui/certificate_trust.h b/shell/browser/ui/certificate_trust.h
index e7962b5de95..02bd8987bee 100644
--- a/shell/browser/ui/certificate_trust.h
+++ b/shell/browser/ui/certificate_trust.h
@@ -14,7 +14,7 @@
 
 namespace electron {
 class NativeWindow;
-}  // namespace electron
+}
 
 namespace certificate_trust {
 
diff --git a/shell/browser/ui/devtools_ui.h b/shell/browser/ui/devtools_ui.h
index 1bbde643a84..8ec356e2ec9 100644
--- a/shell/browser/ui/devtools_ui.h
+++ b/shell/browser/ui/devtools_ui.h
@@ -11,8 +11,6 @@
 
 namespace electron {
 
-class BrowserContext;
-
 class DevToolsUI : public content::WebUIController {
  public:
   explicit DevToolsUI(content::BrowserContext* browser_context,
diff --git a/shell/browser/ui/tray_icon_gtk.h b/shell/browser/ui/tray_icon_gtk.h
index 41193af329d..3ef9db953da 100644
--- a/shell/browser/ui/tray_icon_gtk.h
+++ b/shell/browser/ui/tray_icon_gtk.h
@@ -11,10 +11,6 @@
 #include "shell/browser/ui/tray_icon.h"
 #include "ui/views/linux_ui/status_icon_linux.h"
 
-namespace views {
-class StatusIconLinux;
-}
-
 namespace electron {
 
 class TrayIconGtk : public TrayIcon, public views::StatusIconLinux::Delegate {
diff --git a/shell/browser/web_contents_preferences.h b/shell/browser/web_contents_preferences.h
index 15eda7396a2..4e28252387c 100644
--- a/shell/browser/web_contents_preferences.h
+++ b/shell/browser/web_contents_preferences.h
@@ -15,10 +15,6 @@ namespace base {
 class CommandLine;
 }
 
-namespace content {
-struct WebPreferences;
-}
-
 namespace gin_helper {
 class Dictionary;
 }
diff --git a/shell/browser/zoom_level_delegate.cc b/shell/browser/zoom_level_delegate.cc
index c446b270aa9..a2c340a2121 100644
--- a/shell/browser/zoom_level_delegate.cc
+++ b/shell/browser/zoom_level_delegate.cc
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "base/bind.h"
+#include "base/files/file_path.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/values.h"
 #include "components/prefs/json_pref_store.h"
diff --git a/shell/browser/zoom_level_delegate.h b/shell/browser/zoom_level_delegate.h
index 773cf10c7de..4d6b1051d48 100644
--- a/shell/browser/zoom_level_delegate.h
+++ b/shell/browser/zoom_level_delegate.h
@@ -7,7 +7,6 @@
 
 #include <string>
 
-#include "base/files/file_path.h"
 #include "base/macros.h"
 #include "components/prefs/pref_service.h"
 #include "content/public/browser/host_zoom_map.h"
@@ -15,7 +14,8 @@
 
 namespace base {
 class DictionaryValue;
-}
+class FilePath;
+}  // namespace base
 
 class PrefRegistrySimple;
 
diff --git a/shell/common/api/electron_api_native_image_win.cc b/shell/common/api/electron_api_native_image_win.cc
index 7220e7fecbd..b77a0492266 100644
--- a/shell/common/api/electron_api_native_image_win.cc
+++ b/shell/common/api/electron_api_native_image_win.cc
@@ -12,7 +12,9 @@
 #include "shell/common/gin_converters/image_converter.h"
 #include "shell/common/gin_helper/promise.h"
 #include "shell/common/skia_util.h"
+#include "third_party/skia/include/core/SkBitmap.h"
 #include "ui/gfx/icon_util.h"
+#include "ui/gfx/image/image_skia.h"
 
 namespace electron {
 
diff --git a/shell/common/extensions/electron_extensions_client.h b/shell/common/extensions/electron_extensions_client.h
index dac734bc894..6d124028e20 100644
--- a/shell/common/extensions/electron_extensions_client.h
+++ b/shell/common/extensions/electron_extensions_client.h
@@ -17,7 +17,6 @@ class APIPermissionSet;
 class Extension;
 class PermissionMessageProvider;
 class PermissionIDSet;
-class ScriptingWhitelist;
 class URLPatternSet;
 }  // namespace extensions
 
diff --git a/shell/common/gin_converters/blink_converter.h b/shell/common/gin_converters/blink_converter.h
index b48f820cea8..1330802f0d2 100644
--- a/shell/common/gin_converters/blink_converter.h
+++ b/shell/common/gin_converters/blink_converter.h
@@ -17,7 +17,6 @@ class WebMouseEvent;
 class WebMouseWheelEvent;
 class WebKeyboardEvent;
 struct DeviceEmulationParams;
-struct WebSize;
 }  // namespace blink
 
 namespace gin {
diff --git a/shell/common/gin_converters/extension_converter.h b/shell/common/gin_converters/extension_converter.h
index e108e54f72e..e0d566f6801 100644
--- a/shell/common/gin_converters/extension_converter.h
+++ b/shell/common/gin_converters/extension_converter.h
@@ -11,7 +11,7 @@
 
 namespace extensions {
 class Extension;
-}  // namespace extensions
+}
 
 namespace gin {
 
diff --git a/shell/common/gin_converters/frame_converter.h b/shell/common/gin_converters/frame_converter.h
index 3a500dae893..4916054175e 100644
--- a/shell/common/gin_converters/frame_converter.h
+++ b/shell/common/gin_converters/frame_converter.h
@@ -9,7 +9,7 @@
 
 namespace content {
 class RenderFrameHost;
-}  // namespace content
+}
 
 namespace gin {
 
diff --git a/shell/common/gin_converters/net_converter.h b/shell/common/gin_converters/net_converter.h
index a90e3dd90ec..2341202e1bd 100644
--- a/shell/common/gin_converters/net_converter.h
+++ b/shell/common/gin_converters/net_converter.h
@@ -13,14 +13,8 @@
 #include "services/network/public/mojom/fetch_api.mojom.h"
 #include "shell/browser/net/cert_verifier_client.h"
 
-namespace base {
-class DictionaryValue;
-class ListValue;
-}  // namespace base
-
 namespace net {
 class AuthChallengeInfo;
-class URLRequest;
 class X509Certificate;
 class HttpResponseHeaders;
 struct CertPrincipal;
diff --git a/shell/common/gin_helper/wrappable_base.h b/shell/common/gin_helper/wrappable_base.h
index 4ac145edf47..4b70a91fa24 100644
--- a/shell/common/gin_helper/wrappable_base.h
+++ b/shell/common/gin_helper/wrappable_base.h
@@ -9,7 +9,7 @@
 
 namespace gin {
 class Arguments;
-}  // namespace gin
+}
 
 namespace gin_helper {
 
diff --git a/shell/common/heap_snapshot.cc b/shell/common/heap_snapshot.cc
index a0148eea9bd..2f416e6413a 100644
--- a/shell/common/heap_snapshot.cc
+++ b/shell/common/heap_snapshot.cc
@@ -4,7 +4,9 @@
 
 #include "shell/common/heap_snapshot.h"
 
+#include "base/files/file.h"
 #include "v8/include/v8-profiler.h"
+#include "v8/include/v8.h"
 
 namespace {
 
diff --git a/shell/common/heap_snapshot.h b/shell/common/heap_snapshot.h
index 81712ae3a47..ef544aa0530 100644
--- a/shell/common/heap_snapshot.h
+++ b/shell/common/heap_snapshot.h
@@ -5,8 +5,13 @@
 #ifndef SHELL_COMMON_HEAP_SNAPSHOT_H_
 #define SHELL_COMMON_HEAP_SNAPSHOT_H_
 
-#include "base/files/file.h"
-#include "v8/include/v8.h"
+namespace base {
+class File;
+}
+
+namespace v8 {
+class Isolate;
+}
 
 namespace electron {
 
diff --git a/shell/common/node_bindings.cc b/shell/common/node_bindings.cc
index e1c6634306b..6ff87e79620 100644
--- a/shell/common/node_bindings.cc
+++ b/shell/common/node_bindings.cc
@@ -17,6 +17,7 @@
 #include "base/environment.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
+#include "base/single_thread_task_runner.h"
 #include "base/strings/string_split.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_task_runner_handle.h"
diff --git a/shell/common/node_bindings.h b/shell/common/node_bindings.h
index 3af029e387e..4cbc888b372 100644
--- a/shell/common/node_bindings.h
+++ b/shell/common/node_bindings.h
@@ -10,12 +10,11 @@
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
-#include "base/single_thread_task_runner.h"
 #include "uv.h"  // NOLINT(build/include_directory)
 #include "v8/include/v8.h"
 
 namespace base {
-class MessageLoop;
+class SingleThreadTaskRunner;
 }
 
 namespace node {
diff --git a/shell/common/node_util.h b/shell/common/node_util.h
index 4db7d80092d..a27f224918d 100644
--- a/shell/common/node_util.h
+++ b/shell/common/node_util.h
@@ -11,7 +11,7 @@
 
 namespace node {
 class Environment;
-}  // namespace node
+}
 
 namespace electron {
 
diff --git a/shell/common/process_util.cc b/shell/common/process_util.cc
index 862363e5449..c253af58fc9 100644
--- a/shell/common/process_util.cc
+++ b/shell/common/process_util.cc
@@ -6,6 +6,7 @@
 
 #include "gin/dictionary.h"
 #include "shell/common/gin_converters/callback_converter.h"
+#include "shell/common/node_includes.h"
 
 namespace electron {
 
diff --git a/shell/common/process_util.h b/shell/common/process_util.h
index 6d8584e528b..e66430758c2 100644
--- a/shell/common/process_util.h
+++ b/shell/common/process_util.h
@@ -7,7 +7,9 @@
 
 #include <string>
 
-#include "shell/common/node_includes.h"
+namespace node {
+class Environment;
+}
 
 namespace electron {
 
diff --git a/shell/common/skia_util.cc b/shell/common/skia_util.cc
index ac260fbf722..ad0d8c00c95 100644
--- a/shell/common/skia_util.cc
+++ b/shell/common/skia_util.cc
@@ -4,6 +4,7 @@
 
 #include <string>
 
+#include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/strings/pattern.h"
 #include "base/strings/string_util.h"
diff --git a/shell/common/skia_util.h b/shell/common/skia_util.h
index 720cebdcea2..2c21b40f9b8 100644
--- a/shell/common/skia_util.h
+++ b/shell/common/skia_util.h
@@ -5,7 +5,13 @@
 #ifndef SHELL_COMMON_SKIA_UTIL_H_
 #define SHELL_COMMON_SKIA_UTIL_H_
 
-#include "ui/gfx/image/image_skia.h"
+namespace base {
+class FilePath;
+}
+
+namespace gfx {
+class ImageSkia;
+}
 
 namespace electron {
 
diff --git a/shell/renderer/browser_exposed_renderer_interfaces.h b/shell/renderer/browser_exposed_renderer_interfaces.h
index a35745e7b41..97e32ba7687 100644
--- a/shell/renderer/browser_exposed_renderer_interfaces.h
+++ b/shell/renderer/browser_exposed_renderer_interfaces.h
@@ -7,11 +7,11 @@
 
 namespace mojo {
 class BinderMap;
-}  // namespace mojo
+}
 
 namespace electron {
 class RendererClientBase;
-}  // namespace electron
+}
 
 class ChromeContentRendererClient;
 
diff --git a/shell/renderer/electron_render_frame_observer.cc b/shell/renderer/electron_render_frame_observer.cc
index 9b928f427a8..0f10e16fed2 100644
--- a/shell/renderer/electron_render_frame_observer.cc
+++ b/shell/renderer/electron_render_frame_observer.cc
@@ -21,6 +21,7 @@
 #include "shell/common/gin_helper/microtasks_scope.h"
 #include "shell/common/options_switches.h"
 #include "shell/common/world_ids.h"
+#include "shell/renderer/renderer_client_base.h"
 #include "third_party/blink/public/common/browser_interface_broker_proxy.h"
 #include "third_party/blink/public/common/web_preferences/web_preferences.h"
 #include "third_party/blink/public/platform/web_isolated_world_info.h"
diff --git a/shell/renderer/electron_render_frame_observer.h b/shell/renderer/electron_render_frame_observer.h
index 03f0b82397d..5366bf256e3 100644
--- a/shell/renderer/electron_render_frame_observer.h
+++ b/shell/renderer/electron_render_frame_observer.h
@@ -9,15 +9,12 @@
 
 #include "content/public/renderer/render_frame_observer.h"
 #include "ipc/ipc_platform_file.h"
-#include "shell/renderer/renderer_client_base.h"
 #include "third_party/blink/public/web/web_local_frame.h"
 
-namespace base {
-class ListValue;
-}
-
 namespace electron {
 
+class RendererClientBase;
+
 // Helper class to forward the messages to the client.
 class ElectronRenderFrameObserver : public content::RenderFrameObserver {
  public:
diff --git a/shell/renderer/electron_renderer_client.h b/shell/renderer/electron_renderer_client.h
index 2ecee5ba4c0..9b2f2e591d5 100644
--- a/shell/renderer/electron_renderer_client.h
+++ b/shell/renderer/electron_renderer_client.h
@@ -13,7 +13,7 @@
 
 namespace node {
 class Environment;
-}  // namespace node
+}
 
 namespace electron {
 
diff --git a/shell/renderer/electron_sandboxed_renderer_client.cc b/shell/renderer/electron_sandboxed_renderer_client.cc
index e31221f3b3c..dcd74811546 100644
--- a/shell/renderer/electron_sandboxed_renderer_client.cc
+++ b/shell/renderer/electron_sandboxed_renderer_client.cc
@@ -11,6 +11,7 @@
 #include "base/files/file_path.h"
 #include "base/path_service.h"
 #include "base/process/process_handle.h"
+#include "base/process/process_metrics.h"
 #include "content/public/renderer/render_frame.h"
 #include "electron/buildflags/buildflags.h"
 #include "shell/common/api/electron_bindings.h"
diff --git a/shell/renderer/electron_sandboxed_renderer_client.h b/shell/renderer/electron_sandboxed_renderer_client.h
index fe739eb29e7..8d81b89ff12 100644
--- a/shell/renderer/electron_sandboxed_renderer_client.h
+++ b/shell/renderer/electron_sandboxed_renderer_client.h
@@ -8,9 +8,16 @@
 #include <set>
 #include <string>
 
-#include "base/process/process_metrics.h"
 #include "shell/renderer/renderer_client_base.h"
 
+namespace base {
+class ProcessMetrics;
+}
+
+namespace blink {
+class WebLocalFrame;
+}
+
 namespace electron {
 
 class ElectronSandboxedRendererClient : public RendererClientBase {
diff --git a/shell/renderer/guest_view_container.cc b/shell/renderer/guest_view_container.cc
index 770c3787345..07c2ec81da0 100644
--- a/shell/renderer/guest_view_container.cc
+++ b/shell/renderer/guest_view_container.cc
@@ -10,6 +10,7 @@
 #include "base/bind.h"
 #include "base/lazy_instance.h"
 #include "base/threading/thread_task_runner_handle.h"
+#include "content/public/renderer/render_frame.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace electron {
diff --git a/shell/renderer/guest_view_container.h b/shell/renderer/guest_view_container.h
index 7b34a97fc5b..a8eb00a1120 100644
--- a/shell/renderer/guest_view_container.h
+++ b/shell/renderer/guest_view_container.h
@@ -6,7 +6,10 @@
 #define SHELL_RENDERER_GUEST_VIEW_CONTAINER_H_
 
 #include "base/callback.h"
-#include "content/public/renderer/render_frame.h"
+
+namespace content {
+class RenderFrame;
+}
 
 namespace gfx {
 class Size;
diff --git a/shell/renderer/renderer_client_base.cc b/shell/renderer/renderer_client_base.cc
index 38f0f613c35..1ea78896052 100644
--- a/shell/renderer/renderer_client_base.cc
+++ b/shell/renderer/renderer_client_base.cc
@@ -41,6 +41,7 @@
 #include "third_party/blink/public/web/blink.h"
 #include "third_party/blink/public/web/web_custom_element.h"  // NOLINT(build/include_alpha)
 #include "third_party/blink/public/web/web_frame_widget.h"
+#include "third_party/blink/public/web/web_local_frame.h"
 #include "third_party/blink/public/web/web_plugin_params.h"
 #include "third_party/blink/public/web/web_script_source.h"
 #include "third_party/blink/public/web/web_security_policy.h"
diff --git a/shell/renderer/renderer_client_base.h b/shell/renderer/renderer_client_base.h
index b2c5f9602e2..904b43e9dbe 100644
--- a/shell/renderer/renderer_client_base.h
+++ b/shell/renderer/renderer_client_base.h
@@ -13,7 +13,6 @@
 #include "electron/buildflags/buildflags.h"
 #include "printing/buildflags/buildflags.h"
 #include "shell/common/gin_helper/dictionary.h"
-#include "third_party/blink/public/web/web_local_frame.h"
 // In SHARED_INTERMEDIATE_DIR.
 #include "widevine_cdm_version.h"  // NOLINT(build/include_directory)
 
@@ -32,19 +31,18 @@
 class SpellCheck;
 #endif
 
+namespace blink {
+class WebLocalFrame;
+}
+
 #if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
 namespace extensions {
 class ExtensionsClient;
 }
-namespace content {
-struct WebPluginInfo;
-}
 #endif
 
 namespace electron {
 
-class ElectronApiServiceImpl;
-
 #if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
 class ElectronExtensionsRendererClient;
 #endif
diff --git a/shell/utility/electron_content_utility_client.h b/shell/utility/electron_content_utility_client.h
index 5dd21be77d8..ca057de4411 100644
--- a/shell/utility/electron_content_utility_client.h
+++ b/shell/utility/electron_content_utility_client.h
@@ -18,7 +18,7 @@ class PrintingHandler;
 
 namespace mojo {
 class ServiceFactory;
-}  // namespace mojo
+}
 
 namespace electron {