--- a/content/base/public/nsDeprecatedOperationList.h
+++ b/content/base/public/nsDeprecatedOperationList.h
@@ -1,11 +1,12 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsIDocument.h"
 
 /*
  * This file contains the list of deprecated DOM operations.  It is
  * designed to be used as input to the C preprocessor *only*.
  */
 
 DEPRECATED_OPERATION(GetAttributeNode)
 DEPRECATED_OPERATION(SetAttributeNode)

--- a/content/base/src/nsGkAtomList.h
+++ b/content/base/src/nsGkAtomList.h
@@ -1,13 +1,12 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
 // IWYU pragma: private, include "nsGkAtoms.h"
 
 /*
   This file contains the list of all atoms used by gklayout;
   see nsGkAtoms for access to the atoms.
 */
 
 /*

--- a/content/events/public/nsVKList.h
+++ b/content/events/public/nsVKList.h
@@ -1,13 +1,14 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsGUIEvent.h"
 
 /**
  * This header file defines all DOM keys which are defined in nsIDOMKeyEvent.
  * You must define NS_DEFINE_VK macro before including this.
  *
  * It must have two arguments, (aDOMKeyName, aDOMKeyCode)
  * aDOMKeyName is a key name in DOM.
  * aDOMKeyCode is one of nsIDOMKeyEvent::DOM_VK_*.

--- a/content/html/content/src/HTMLBodyElement.cpp
+++ b/content/html/content/src/HTMLBodyElement.cpp
@@ -536,17 +536,17 @@ HTMLBodyElement::IsEventAttributeName(ns
     nsGlobalWindow* globalWin = nsGlobalWindow::FromSupports(supports);        \
     return globalWin->SetOn##name_(handler, error);                            \
   }                                                                            \
   FORWARDED_EVENT_HELPER(name_, HTMLBodyElement, type_, type_*)
 #define WINDOW_EVENT(name_, id_, type_, struct_)                               \
   WINDOW_EVENT_HELPER(name_, EventHandlerNonNull)
 #define BEFOREUNLOAD_EVENT(name_, id_, type_, struct_)                         \
   WINDOW_EVENT_HELPER(name_, BeforeUnloadEventHandlerNonNull)
-#include "nsEventNameList.h"
+#include "nsEventNameList.h" // IWYU pragma: keep
 #undef BEFOREUNLOAD_EVENT
 #undef WINDOW_EVENT
 #undef WINDOW_EVENT_HELPER
 #undef ERROR_EVENT
 #undef FORWARDED_EVENT
 #undef FORWARDED_EVENT_HELPER
 #undef EVENT
 

--- a/content/html/content/src/HTMLBodyElement.h
+++ b/content/html/content/src/HTMLBodyElement.h
@@ -60,17 +60,17 @@ public:
   NS_IMETHOD SetOn##name_(JSContext *cx, const JS::Value &v);
 #define WINDOW_EVENT_HELPER(name_, type_)                               \
   type_* GetOn##name_();                                                \
   void SetOn##name_(type_* handler, ErrorResult& error);
 #define WINDOW_EVENT(name_, id_, type_, struct_)                        \
   WINDOW_EVENT_HELPER(name_, EventHandlerNonNull)
 #define BEFOREUNLOAD_EVENT(name_, id_, type_, struct_)                  \
   WINDOW_EVENT_HELPER(name_, BeforeUnloadEventHandlerNonNull)
-#include "nsEventNameList.h"
+#include "nsEventNameList.h" // IWYU pragma: keep
 #undef BEFOREUNLOAD_EVENT
 #undef WINDOW_EVENT
 #undef WINDOW_EVENT_HELPER
 #undef FORWARDED_EVENT
 #undef EVENT
 
   void GetText(nsString& aText)
   {

--- a/content/html/content/src/HTMLFrameSetElement.cpp
+++ b/content/html/content/src/HTMLFrameSetElement.cpp
@@ -395,17 +395,17 @@ HTMLFrameSetElement::IsEventAttributeNam
     nsGlobalWindow* globalWin = nsGlobalWindow::FromSupports(supports);        \
     return globalWin->SetOn##name_(handler, error);                            \
   }                                                                            \
   FORWARDED_EVENT_HELPER(name_, HTMLFrameSetElement, type_, type_*)
 #define WINDOW_EVENT(name_, id_, type_, struct_)                               \
   WINDOW_EVENT_HELPER(name_, EventHandlerNonNull)
 #define BEFOREUNLOAD_EVENT(name_, id_, type_, struct_)                         \
   WINDOW_EVENT_HELPER(name_, BeforeUnloadEventHandlerNonNull)
-#include "nsEventNameList.h"
+#include "nsEventNameList.h" // IWYU pragma: keep
 #undef BEFOREUNLOAD_EVENT
 #undef WINDOW_EVENT
 #undef WINDOW_EVENT_HELPER
 #undef ERROR_EVENT
 #undef FORWARDED_EVENT
 #undef FORWARDED_EVENT_HELPER
 #undef EVENT
 

--- a/content/html/content/src/HTMLFrameSetElement.h
+++ b/content/html/content/src/HTMLFrameSetElement.h
@@ -90,17 +90,17 @@ public:
   NS_IMETHOD SetOn##name_(JSContext *cx, const JS::Value &v);
 #define WINDOW_EVENT_HELPER(name_, type_)                               \
   type_* GetOn##name_();                                                \
   void SetOn##name_(type_* handler, ErrorResult& error);
 #define WINDOW_EVENT(name_, id_, type_, struct_)                        \
   WINDOW_EVENT_HELPER(name_, EventHandlerNonNull)
 #define BEFOREUNLOAD_EVENT(name_, id_, type_, struct_)                  \
   WINDOW_EVENT_HELPER(name_, BeforeUnloadEventHandlerNonNull)
-#include "nsEventNameList.h"
+#include "nsEventNameList.h" // IWYU pragma: keep
 #undef BEFOREUNLOAD_EVENT
 #undef WINDOW_EVENT
 #undef WINDOW_EVENT_HELPER
 #undef FORWARDED_EVENT
 #undef EVENT
 
   // These override the SetAttr methods in nsGenericHTMLElement (need
   // both here to silence compiler warnings).

--- a/content/html/content/src/nsGenericHTMLElement.cpp
+++ b/content/html/content/src/nsGenericHTMLElement.cpp
@@ -815,17 +815,17 @@ nsGenericHTMLElement::GetEventListenerMa
   if ((mNodeInfo->Equals(nsGkAtoms::body) ||
        mNodeInfo->Equals(nsGkAtoms::frameset)) &&
       // We only forward some event attributes from body/frameset to window
       (0
 #define EVENT(name_, id_, type_, struct_) /* nothing */
 #define FORWARDED_EVENT(name_, id_, type_, struct_) \
        || nsGkAtoms::on##name_ == aAttrName
 #define WINDOW_EVENT FORWARDED_EVENT
-#include "nsEventNameList.h"
+#include "nsEventNameList.h" // IWYU pragma: keep
 #undef WINDOW_EVENT
 #undef FORWARDED_EVENT
 #undef EVENT
        )
       ) {
     nsPIDOMWindow *win;
 
     // If we have a document, and it has a window, add the event
@@ -923,17 +923,17 @@ nsGenericHTMLElement::SetOn##name_(Event
     if (handler) {                                                            \
       errorHandler = new OnErrorEventHandlerNonNull(handler);                 \
     }                                                                         \
     return globalWin->SetOn##name_(errorHandler, error);                      \
   }                                                                           \
                                                                               \
   return nsINode::SetOn##name_(handler, error);                               \
 }
-#include "nsEventNameList.h"
+#include "nsEventNameList.h" // IWYU pragma: keep
 #undef ERROR_EVENT
 #undef FORWARDED_EVENT
 #undef EVENT
 
 nsresult
 nsGenericHTMLElement::SetAttr(int32_t aNameSpaceID, nsIAtom* aName,
                               nsIAtom* aPrefix, const nsAString& aValue,
                               bool aNotify)

--- a/content/html/content/src/nsGenericHTMLElement.h
+++ b/content/html/content/src/nsGenericHTMLElement.h
@@ -242,17 +242,17 @@ public:
   void SetOn##name_(mozilla::dom::EventHandlerNonNull* handler,               \
                     mozilla::ErrorResult& error);
 #define ERROR_EVENT(name_, id_, type_, struct_)                               \
   using nsINode::GetOn##name_;                                                \
   using nsINode::SetOn##name_;                                                \
   already_AddRefed<mozilla::dom::EventHandlerNonNull> GetOn##name_();         \
   void SetOn##name_(mozilla::dom::EventHandlerNonNull* handler,               \
                     mozilla::ErrorResult& error);
-#include "nsEventNameList.h"
+#include "nsEventNameList.h" // IWYU pragma: keep
 #undef ERROR_EVENT
 #undef FORWARDED_EVENT
 #undef EVENT
   void GetClassName(nsString& aClassName)
   {
     GetAttr(kNameSpaceID_None, nsGkAtoms::_class, aClassName);
   }
   void SetClassName(const nsAString& aClassName)

--- a/dom/base/nsDOMClassInfoClasses.h
+++ b/dom/base/nsDOMClassInfoClasses.h
@@ -1,12 +1,13 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsDOMClassInfoID.h"
 
 DOMCI_CLASS(Window)
 DOMCI_CLASS(Location)
 DOMCI_CLASS(DOMPrototype)
 DOMCI_CLASS(DOMConstructor)
 
 // Core classes
 DOMCI_CLASS(DOMException)

--- a/tools/profiler/GeckoProfilerImpl.h
+++ b/tools/profiler/GeckoProfilerImpl.h
@@ -1,12 +1,13 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "GeckoProfiler.h"
 
 #ifndef TOOLS_SPS_SAMPLER_H_
 #define TOOLS_SPS_SAMPLER_H_
 
 #include <stdlib.h>
 #include <signal.h>
 #include <stdarg.h>
 #include <algorithm>

--- a/xpcom/base/ErrorList.h
+++ b/xpcom/base/ErrorList.h
@@ -1,8 +1,9 @@
+// IWYU pragma: private, include "nsError.h"
 /* Helper file for nsError.h, via preprocessor magic */
   /* Standard "it worked" return value */
   ERROR(NS_OK,  0),
 
   /* ======================================================================= */
   /* Core errors, not part of any modules */
   /* ======================================================================= */
   ERROR(NS_ERROR_BASE,                          0xC1F30000),

--- a/xpcom/base/Makefile.in
+++ b/xpcom/base/Makefile.in
@@ -38,17 +38,18 @@ LOCAL_INCLUDES += \
 		-I$(srcdir)/../build \
 		-I$(topsrcdir)/xpcom/ds \
 		$(NULL)
 
 # We generate ErrorListCxxDefines.h from ErrorList.h using regex.  The -n option
 # suppresses printing the pattern space, and the p at the end prints it anyway,
 # so we don't print lines that don't match the pattern to start with.
 ErrorListCxxDefines.h: ErrorList.h Makefile
-		sed -n 's/.*ERROR(\([A-Z_0-9]*\).*/#define \1 nsresult::\1/p' < $< > $@
+	echo '// IWYU pragma: private, include "nsError.h"' > $@
+	sed -n 's/.*ERROR(\([A-Z_0-9]*\).*/#define \1 nsresult::\1/p' < $< >> $@
 
 ErrorListCDefines.h: ErrorList.h Makefile
 		sed 's/.*ERROR(\([A-Z_0-9]*\),\( *\)\(.*\))[^)]*/#define \1 \2((nsresult)(\3))/' < $< > $@
 
 GARBAGE += \
 	ErrorListCxxDefines.h \
 	ErrorListCDefines.h \
 	$(NULL)

--- a/xpcom/base/nsISupportsBase.h
+++ b/xpcom/base/nsISupportsBase.h
@@ -1,11 +1,12 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsISupports.h"
 
 #ifndef nsISupportsBase_h__
 #define nsISupportsBase_h__
 
 #ifndef nscore_h___
 #include "nscore.h"
 #endif
 

--- a/xpcom/build/ServiceList.h
+++ b/xpcom/build/ServiceList.h
@@ -1,12 +1,11 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
 // IWYU pragma: private, include "mozilla/Services.h"
 
 #ifdef ACCESSIBILITY
 MOZ_SERVICE(AccessibilityService, nsIAccessibilityService, "@mozilla.org/accessibilityService;1")
 #endif
 MOZ_SERVICE(ChromeRegistryService, nsIChromeRegistry, "@mozilla.org/chrome/chrome-registry;1")
 MOZ_SERVICE(ToolkitChromeRegistryService, nsIToolkitChromeRegistry, "@mozilla.org/chrome/chrome-registry;1")
 MOZ_SERVICE(XULChromeRegistryService, nsIXULChromeRegistry, "@mozilla.org/chrome/chrome-registry;1")

--- a/xpcom/glue/nsISupportsImpl.h
+++ b/xpcom/glue/nsISupportsImpl.h
@@ -1,12 +1,13 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsISupports.h"
 
 
 #ifndef nsISupportsImpl_h__
 #define nsISupportsImpl_h__
 
 #ifndef nscore_h___
 #include "nscore.h"
 #endif

--- a/xpcom/string/public/nsAString.h
+++ b/xpcom/string/public/nsAString.h
@@ -1,13 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsStringGlue.h"
 
 #ifndef nsAString_h___
 #define nsAString_h___
 
 #ifndef nsStringFwd_h___
 #include "nsStringFwd.h"
 #endif
 

--- a/xpcom/string/public/nsReadableUtils.h
+++ b/xpcom/string/public/nsReadableUtils.h
@@ -1,12 +1,13 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsStringGlue.h"
 
 #ifndef nsReadableUtils_h___
 #define nsReadableUtils_h___
 
   /**
    * I guess all the routines in this file are all mis-named.
    * According to our conventions, they should be |NS_xxx|.
    */

--- a/xpcom/string/public/nsString.h
+++ b/xpcom/string/public/nsString.h
@@ -1,13 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsStringGlue.h"
 
 #ifndef nsString_h___
 #define nsString_h___
 
 #include "mozilla/Attributes.h"
 
 #ifndef nsSubstring_h___
 #include "nsSubstring.h"

--- a/xpcom/string/public/nsTDependentSubstring.h
+++ b/xpcom/string/public/nsTDependentSubstring.h
@@ -1,15 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-// IWYU pragma: private, include "nsDependentSubstring.h"
+// IWYU pragma: private, include "nsStringGlue.h"
 
   /**
    * nsTDependentSubstring_CharT
    *
    * A string class which wraps an external array of string characters. It
    * is the client code's responsibility to ensure that the external buffer
    * remains valid for a long as the string is alive.
    *

--- a/xpcom/string/public/nsTString.h
+++ b/xpcom/string/public/nsTString.h
@@ -1,15 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-// IWYU pragma: private, include "nsString.h"
+// IWYU pragma: private, include "nsStringGlue.h"
 
   /**
    * This is the canonical null-terminated string class.  All subclasses
    * promise null-terminated storage.  Instances of this class allocate
    * strings on the heap.
    *
    * NAMES:
    *   nsString for wide characters

--- a/xpcom/string/public/nsTSubstring.h
+++ b/xpcom/string/public/nsTSubstring.h
@@ -1,15 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-// IWYU pragma: private, include "nsAString.h"
+// IWYU pragma: private, include "nsStringGlue.h"
 
 #include "mozilla/MemoryReporting.h"
 
 #ifndef MOZILLA_INTERNAL_API
 #error Cannot use internal string classes without MOZILLA_INTERNAL_API defined. Use the frozen header nsStringAPI.h instead.
 #endif
 
   /**

--- a/xpcom/string/public/nsTSubstringTuple.h
+++ b/xpcom/string/public/nsTSubstringTuple.h
@@ -1,15 +1,14 @@
 //* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-// IWYU pragma: private, include "nsSubstringTuple.h"
+// IWYU pragma: private, include "nsStringGlue.h"
 
   /**
    * nsTSubstringTuple_CharT
    *
    * Represents a tuple of string fragments.  Built as a recursive binary tree.
    * It is used to implement the concatenation of two or more string objects.
    *
    * NOTE: This class is a private implementation detail and should never be 

--- a/xpcom/string/public/string-template-def-char.h
+++ b/xpcom/string/public/string-template-def-char.h
@@ -1,15 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-// IWYU pragma: private, include "nsString.h"
+// IWYU pragma: private, include "nsStringGlue.h"
 
 #define CharT                               char
 #define CharT_is_char                       1
 #define nsTAString_IncompatibleCharT        nsAString
 #define nsTString_CharT                     nsCString
 #define nsTFixedString_CharT                nsFixedCString
 #define nsTAutoString_CharT                 nsAutoCString
 #define nsTSubstring_CharT                  nsACString

--- a/xpcom/string/public/string-template-def-unichar.h
+++ b/xpcom/string/public/string-template-def-unichar.h
@@ -1,15 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-// IWYU pragma: private, include "nsString.h"
+// IWYU pragma: private, include "nsStringGlue.h"
 
 #define CharT                               PRUnichar
 #define CharT_is_PRUnichar                  1
 #define nsTAString_IncompatibleCharT        nsACString
 #define nsTString_CharT                     nsString
 #define nsTFixedString_CharT                nsFixedString
 #define nsTAutoString_CharT                 nsAutoString
 #define nsTSubstring_CharT                  nsAString

--- a/xpcom/string/public/string-template-undef.h
+++ b/xpcom/string/public/string-template-undef.h
@@ -1,13 +1,14 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// IWYU pragma: private, include "nsStringGlue.h"
 
 #undef CharT
 #undef CharT_is_PRUnichar
 #undef CharT_is_char
 #undef nsTAString_IncompatibleCharT
 #undef nsTString_CharT
 #undef nsTFixedString_CharT
 #undef nsTAutoString_CharT