diff --git a/caps/nsScriptSecurityManager.cpp b/caps/nsScriptSecurityManager.cpp
index d155116bb766..e66d4cdb5ee3 100644
--- a/caps/nsScriptSecurityManager.cpp
+++ b/caps/nsScriptSecurityManager.cpp
@@ -390,7 +390,7 @@ nsScriptSecurityManager::GetChannelURIPrincipal(nsIChannel* aChannel,
   MOZ_ASSERT(aChannel, "Must have channel!");
 
   // Get the principal from the URI.  Make sure this does the same thing
-  // as nsDocument::Reset and XULDocument::StartDocumentLoad.
+  // as nsIDocument::Reset and XULDocument::StartDocumentLoad.
   nsCOMPtr<nsIURI> uri;
   nsresult rv = NS_GetFinalChannelURI(aChannel, getter_AddRefs(uri));
   NS_ENSURE_SUCCESS(rv, rv);
diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 2b14a2e1d8fc..47f8cf69b565 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -2476,7 +2476,7 @@ void nsDocShell::MaybeCreateInitialClientSource(nsIPrincipal* aPrincipal) {
   // Sometimes there is no principal available when we are called from
   // CreateAboutBlankContentViewer.  For example, sometimes the principal
   // is only extracted from the load context after the document is created
-  // in nsDocument::ResetToURI().  Ideally we would do something similar
+  // in nsIDocument::ResetToURI().  Ideally we would do something similar
   // here, but for now lets just avoid the issue by not preallocating the
   // client.
   if (!principal) {
@@ -10885,7 +10885,7 @@ bool nsDocShell::OnLoadingSite(nsIChannel* aChannel, bool aFireOnLocationChange,
   // If this a redirect, use the final url (uri)
   // else use the original url
   //
-  // Note that this should match what documents do (see nsDocument::Reset).
+  // Note that this should match what documents do (see nsIDocument::Reset).
   NS_GetFinalChannelURI(aChannel, getter_AddRefs(uri));
   NS_ENSURE_TRUE(uri, false);
 
diff --git a/docshell/base/nsPingListener.cpp b/docshell/base/nsPingListener.cpp
index 2d2a6270aff4..7cdc3e2f1047 100644
--- a/docshell/base/nsPingListener.cpp
+++ b/docshell/base/nsPingListener.cpp
@@ -16,7 +16,7 @@
 #include "nsIProtocolHandler.h"
 #include "nsIUploadChannel2.h"
 
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsNetUtil.h"
 #include "nsStreamUtils.h"
 #include "nsStringStream.h"
diff --git a/dom/animation/KeyframeUtils.cpp b/dom/animation/KeyframeUtils.cpp
index 303b6abc6a9c..d45c75c3b110 100644
--- a/dom/animation/KeyframeUtils.cpp
+++ b/dom/animation/KeyframeUtils.cpp
@@ -27,7 +27,7 @@
 #include "nsCSSPropertyIDSet.h"
 #include "nsCSSProps.h"
 #include "nsCSSPseudoElements.h"  // For CSSPseudoElementType
-#include "nsDocument.h"  // For nsDocument::AreWebAnimationsImplicitKeyframesEnabled
+#include "nsIDocument.h"  // For nsIDocument::AreWebAnimationsImplicitKeyframesEnabled
 #include "nsIScriptError.h"
 #include "nsTArray.h"
 #include <algorithm>  // For std::stable_sort, std::min
@@ -224,7 +224,7 @@ static void DistributeRange(const Range<Keyframe>& aRange);
     return keyframes;
   }
 
-  if (!nsDocument::AreWebAnimationsImplicitKeyframesEnabled(aCx, nullptr) &&
+  if (!nsIDocument::AreWebAnimationsImplicitKeyframesEnabled(aCx, nullptr) &&
       HasImplicitKeyframeValues(keyframes, aDocument)) {
     keyframes.Clear();
     aRv.Throw(NS_ERROR_DOM_ANIM_MISSING_PROPS_ERR);
diff --git a/dom/base/ChildIterator.cpp b/dom/base/ChildIterator.cpp
index 2b4d4fb93876..2dfc3491ed9c 100644
--- a/dom/base/ChildIterator.cpp
+++ b/dom/base/ChildIterator.cpp
@@ -12,7 +12,7 @@
 #include "nsIAnonymousContentCreator.h"
 #include "nsIFrame.h"
 #include "nsCSSAnonBoxes.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 
 namespace mozilla {
 namespace dom {
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index 4c8df302f473..b6e341c57e43 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -22,8 +22,6 @@
 #include "nsWrapperCache.h"
 #include "nsContentUtils.h"
 
-class nsDocument;
-
 namespace mozilla {
 namespace dom {
 
@@ -351,8 +349,6 @@ class CustomElementReactionsStack {
 };
 
 class CustomElementRegistry final : public nsISupports, public nsWrapperCache {
-  // Allow nsDocument to access mCustomDefinitions and mCandidatesMap.
-  friend class ::nsDocument;
 
  public:
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
diff --git a/dom/base/DOMParser.cpp b/dom/base/DOMParser.cpp
index 14b6eb97f92d..4210197b0ddb 100644
--- a/dom/base/DOMParser.cpp
+++ b/dom/base/DOMParser.cpp
@@ -271,10 +271,10 @@ already_AddRefed<DOMParser> DOMParser::CreateWithoutGlobal(ErrorResult& aRv) {
 
 already_AddRefed<nsIDocument> DOMParser::SetUpDocument(DocumentFlavor aFlavor,
                                                        ErrorResult& aRv) {
-  // We should really just use mOwner here, but nsDocument gets confused
+  // We should really just use mOwner here, but nsIDocument gets confused
   // if we pass it a scriptHandlingObject that doesn't QI to
   // nsIScriptGlobalObject, and test_isequalnode.js (an xpcshell test without
-  // a window global) breaks. The correct solution is just to wean nsDocument
+  // a window global) breaks. The correct solution is just to wean nsIDocument
   // off of nsIScriptGlobalObject, but that's a yak to shave another day.
   nsCOMPtr<nsIScriptGlobalObject> scriptHandlingObject =
       do_QueryInterface(mOwner);
diff --git a/dom/base/DocumentOrShadowRoot.cpp b/dom/base/DocumentOrShadowRoot.cpp
index 21f9874b82ad..c67c1a22055c 100644
--- a/dom/base/DocumentOrShadowRoot.cpp
+++ b/dom/base/DocumentOrShadowRoot.cpp
@@ -9,7 +9,7 @@
 #include "mozilla/dom/HTMLInputElement.h"
 #include "mozilla/dom/ShadowRoot.h"
 #include "mozilla/dom/StyleSheetList.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsFocusManager.h"
 #include "nsIRadioVisitor.h"
 #include "nsIFormControl.h"
@@ -476,8 +476,8 @@ nsresult DocumentOrShadowRoot::GetNextRadioButton(
     const nsAString& aName, const bool aPrevious,
     HTMLInputElement* aFocusedRadio, HTMLInputElement** aRadioOut) {
   // XXX Can we combine the HTML radio button method impls of
-  //     nsDocument and nsHTMLFormControl?
-  // XXX Why is HTML radio button stuff in nsDocument, as
+  //     nsIDocument and nsHTMLFormControl?
+  // XXX Why is HTML radio button stuff in nsIDocument, as
   //     opposed to nsHTMLDocument?
   *aRadioOut = nullptr;
 
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index ff3b92147a19..56c373ffabb3 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -76,7 +76,7 @@
 #include "mozilla/TextEvents.h"
 #include "nsNodeUtils.h"
 #include "mozilla/dom/DirectionalityUtils.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsAttrValueOrString.h"
 #include "nsAttrValueInlines.h"
 #include "nsCSSPseudoElements.h"
diff --git a/dom/base/Element.h b/dom/base/Element.h
index 10667a471561..fe3e852be16d 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -59,7 +59,6 @@ class nsGlobalWindowInner;
 class nsGlobalWindowOuter;
 class nsDOMCSSAttributeDeclaration;
 class nsISMILAttr;
-class nsDocument;
 class nsDOMStringMap;
 struct ServoNodeData;
 
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index 53dbe3b22d50..e939877b6dc4 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -61,7 +61,7 @@
 #include "mozilla/InternalMutationEvent.h"
 #include "mozilla/MouseEvents.h"
 #include "nsNodeUtils.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsAttrValueOrString.h"
 #include "nsQueryObject.h"
 #ifdef MOZ_XUL
@@ -1143,7 +1143,7 @@ void FragmentOrElement::DestroyContent() {
   // around the flattened tree.
   //
   // TODO(emilio): I suspect this can be asserted against instead, with a bit of
-  // effort to avoid calling nsDocument::Destroy with a shell...
+  // effort to avoid calling nsIDocument::Destroy with a shell...
   if (IsElement()) {
     AsElement()->ClearServoData();
   }
@@ -1234,7 +1234,7 @@ class ContentUnbinder : public Runnable {
         // observers and they should really see consistent
         // tree state.
         // If this code changes, change the corresponding code in
-        // FragmentOrElement's and nsDocument's unlink impls.
+        // FragmentOrElement's and nsIDocument's unlink impls.
         nsCOMPtr<nsIContent> child = container->GetLastChild();
         container->DisconnectChild(child);
         UnbindSubtree(child);
@@ -1340,7 +1340,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(FragmentOrElement)
     while (tmp->HasChildren()) {
       // Hold a strong ref to the node when we remove it, because we may be
       // the last reference to it.
-      // If this code changes, change the corresponding code in nsDocument's
+      // If this code changes, change the corresponding code in nsIDocument's
       // unlink impl and ContentUnbinder::UnbindSubtree.
       nsCOMPtr<nsIContent> child = tmp->GetLastChild();
       tmp->DisconnectChild(child);
diff --git a/dom/base/InProcessTabChildMessageManager.cpp b/dom/base/InProcessTabChildMessageManager.cpp
index a0ee48b05f56..a9ef37bf1666 100644
--- a/dom/base/InProcessTabChildMessageManager.cpp
+++ b/dom/base/InProcessTabChildMessageManager.cpp
@@ -183,7 +183,7 @@ uint64_t InProcessTabChildMessageManager::ChromeOuterWindowID() {
 }
 
 void InProcessTabChildMessageManager::FireUnloadEvent() {
-  // We're called from nsDocument::MaybeInitializeFinalizeFrameLoaders, so it
+  // We're called from nsIDocument::MaybeInitializeFinalizeFrameLoaders, so it
   // should be safe to run script.
   MOZ_ASSERT(nsContentUtils::IsSafeToRunScript());
 
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 824ef091bb0d..2b3adfb58692 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -111,7 +111,7 @@
 #include "nsCycleCollector.h"
 #include "nsDataHashtable.h"
 #include "nsDocShellCID.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsDOMCID.h"
 #include "mozilla/dom/DataTransfer.h"
 #include "nsDOMJSUtils.h"
diff --git a/dom/base/nsDOMWindowUtils.cpp b/dom/base/nsDOMWindowUtils.cpp
index e402f7c75d70..16f9246b782b 100644
--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -13,7 +13,6 @@
 #include "nsError.h"
 #include "nsQueryContentEventResult.h"
 #include "nsGlobalWindow.h"
-#include "nsIDocument.h"
 #include "nsFocusManager.h"
 #include "nsFrameManager.h"
 #include "nsRefreshDriver.h"
@@ -104,7 +103,7 @@
 #include "nsContentPermissionHelper.h"
 #include "nsCSSPseudoElements.h"  // for CSSPseudoElementType
 #include "nsNetUtil.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "HTMLImageElement.h"
 #include "HTMLCanvasElement.h"
 #include "mozilla/css/ImageLoader.h"
@@ -3793,8 +3792,8 @@ nsDOMWindowUtils::ForceUseCounterFlush(nsINode* aNode) {
 
     // Flush the document and any external documents that it depends on.
     const auto reportKind =
-        nsDocument::UseCounterReportKind::eIncludeExternalResources;
-    static_cast<nsDocument*>(doc.get())->ReportUseCounters(reportKind);
+        nsIDocument::UseCounterReportKind::eIncludeExternalResources;
+    doc->ReportUseCounters(reportKind);
     return NS_OK;
   }
 
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index daf8044c1fd6..7dd368c1946d 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -9,7 +9,7 @@
  */
 
 #include "AudioChannelService.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsIDocumentInlines.h"
 #include "mozilla/AnimationComparator.h"
 #include "mozilla/AntiTrackingCommon.h"
@@ -295,6 +295,11 @@
 #include "mozilla/net/RequestContextService.h"
 #include "StorageAccessPermissionRequest.h"
 
+#define XML_DECLARATION_BITS_DECLARATION_EXISTS (1 << 0)
+#define XML_DECLARATION_BITS_ENCODING_EXISTS (1 << 1)
+#define XML_DECLARATION_BITS_STANDALONE_EXISTS (1 << 2)
+#define XML_DECLARATION_BITS_STANDALONE_YES (1 << 3)
+
 using namespace mozilla;
 using namespace mozilla::dom;
 
@@ -1238,7 +1243,7 @@ struct nsIDocument::FrameRequest {
 // ==================================================================
 // =
 // ==================================================================
-nsIDocument::nsIDocument()
+nsIDocument::nsIDocument(const char* aContentType)
     : nsINode(nullptr),
       DocumentOrShadowRoot(*this),
       mReferrerPolicySet(false),
@@ -1378,6 +1383,7 @@ nsIDocument::nsIDocument()
       mViewportOverflowType(ViewportOverflowType::NoOverflow),
       mSubDocuments(nullptr),
       mHeaderData(nullptr),
+      mPrincipalFlashClassifier(new PrincipalFlashClassifier()),
       mFlashClassification(FlashClassification::Unclassified),
       mBoxObjectTable(nullptr),
       mCurrentOrientationAngle(0),
@@ -1387,27 +1393,22 @@ nsIDocument::nsIDocument()
       mIgnoreOpensDuringUnloadCounter(0),
       mDocLWTheme(Doc_Theme_Uninitialized),
       mSavedResolution(1.0f) {
+  MOZ_LOG(gDocumentLeakPRLog, LogLevel::Debug, ("DOCUMENT %p created", this));
+
   SetIsInDocument();
   SetIsConnected(true);
 
   if (StaticPrefs::layout_css_use_counters_enabled()) {
     mStyleUseCounters.reset(Servo_UseCounters_Create());
   }
-}
 
-nsDocument::nsDocument(const char* aContentType) : nsIDocument() {
   SetContentTypeInternal(nsDependentCString(aContentType));
 
-  MOZ_LOG(gDocumentLeakPRLog, LogLevel::Debug, ("DOCUMENT %p created", this));
-
   // Start out mLastStyleSheetSet as null, per spec
   SetDOMStringToNull(mLastStyleSheetSet);
 
   // void state used to differentiate an empty source from an unselected source
   mPreloadPictureFoundSource.SetIsVoid(true);
-  // For determining if this is a flash document which should be
-  // blocked based on its principal.
-  mPrincipalFlashClassifier = new PrincipalFlashClassifier();
 }
 
 void nsIDocument::ClearAllBoxObjects() {
@@ -1423,21 +1424,6 @@ void nsIDocument::ClearAllBoxObjects() {
   }
 }
 
-nsIDocument::~nsIDocument() {
-  MOZ_ASSERT(mDOMMediaQueryLists.isEmpty(),
-             "must not have media query lists left");
-
-  if (mNodeInfoManager) {
-    mNodeInfoManager->DropDocumentReference();
-  }
-
-  if (mDocGroup) {
-    mDocGroup->RemoveDocument(this);
-  }
-
-  UnlinkOriginalDocumentIfStatic();
-}
-
 bool nsIDocument::IsAboutPage() const {
   nsCOMPtr<nsIPrincipal> principal = NodePrincipal();
   nsCOMPtr<nsIURI> uri;
@@ -1453,7 +1439,7 @@ void nsIDocument::ConstructUbiNode(void* storage) {
   JS::ubi::Concrete<nsIDocument>::construct(storage, this);
 }
 
-nsDocument::~nsDocument() {
+nsIDocument::~nsIDocument() {
   MOZ_LOG(gDocumentLeakPRLog, LogLevel::Debug, ("DOCUMENT %p destroyed", this));
   MOZ_ASSERT(!IsTopLevelContentDocument() || !IsResourceDoc(),
              "Can't be top-level and a resource doc at the same time");
@@ -1638,6 +1624,19 @@ nsDocument::~nsDocument() {
   mPendingTitleChangeEvent.Revoke();
 
   mPlugins.Clear();
+
+  MOZ_ASSERT(mDOMMediaQueryLists.isEmpty(),
+             "must not have media query lists left");
+
+  if (mNodeInfoManager) {
+    mNodeInfoManager->DropDocumentReference();
+  }
+
+  if (mDocGroup) {
+    mDocGroup->RemoveDocument(this);
+  }
+
+  UnlinkOriginalDocumentIfStatic();
 }
 
 NS_INTERFACE_TABLE_HEAD(nsIDocument)
@@ -1754,7 +1753,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(nsIDocument)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mReadyForIdle)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDocumentL10n)
 
-  // Traverse all nsDocument nsCOMPtrs.
+  // Traverse all nsIDocument nsCOMPtrs.
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mParser)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mScriptGlobalObject)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mListenerManager)
@@ -2749,8 +2748,7 @@ nsresult nsIDocument::InitCSP(nsIChannel* aChannel) {
           ("Document is an add-on or CSP header specified %p", this));
 
   nsCOMPtr<nsIContentSecurityPolicy> csp;
-  rv =
-      principal->EnsureCSP(static_cast<nsDocument*>(this), getter_AddRefs(csp));
+  rv = principal->EnsureCSP(this, getter_AddRefs(csp));
   NS_ENSURE_SUCCESS(rv, rv);
 
   // ----- if the doc is an addon, apply its CSP.
@@ -4494,12 +4492,12 @@ void nsIDocument::SetScriptGlobalObject(
   // Some observers need the notification, for example HTMLMediaElement uses
   // it to update internal media resource allocation.
   // When video is loaded via VideoDocument, HTMLMediaElement and MediaDecoder
-  // creation are already done before nsDocument::SetScriptGlobalObject() call.
+  // creation are already done before nsIDocument::SetScriptGlobalObject() call.
   // MediaDecoder decides whether starting decoding is decided based on
   // document's visibility. When the MediaDecoder is created,
-  // nsDocument::SetScriptGlobalObject() is not yet called and document is
+  // nsIDocument::SetScriptGlobalObject() is not yet called and document is
   // hidden state. Therefore the MediaDecoder decides that decoding is
-  // not yet necessary. But soon after nsDocument::SetScriptGlobalObject()
+  // not yet necessary. But soon after nsIDocument::SetScriptGlobalObject()
   // call, the document becomes not hidden. At the time, MediaDecoder needs
   // to know it and needs to start updating decoding.
   if (oldState != mVisibilityState) {
@@ -4616,8 +4614,8 @@ void nsIDocument::MaybeEndOutermostXBLUpdate() {
     } else if (!mInDestructor) {
       if (!mMaybeEndOutermostXBLUpdateRunner) {
         mMaybeEndOutermostXBLUpdateRunner =
-            NewRunnableMethod("nsDocument::MaybeEndOutermostXBLUpdate", this,
-                              &nsDocument::MaybeEndOutermostXBLUpdate);
+            NewRunnableMethod("nsIDocument::MaybeEndOutermostXBLUpdate", this,
+                              &nsIDocument::MaybeEndOutermostXBLUpdate);
       }
       nsContentUtils::AddScriptRunner(mMaybeEndOutermostXBLUpdateRunner);
     }
@@ -7290,7 +7288,7 @@ already_AddRefed<Element> nsIDocument::CreateElem(const nsAString& aName,
   // which would cause an error if we just used true here.
   bool nsAware = aPrefix != nullptr || aNamespaceID != GetDefaultNamespaceID();
   NS_ASSERTION(NS_SUCCEEDED(nsContentUtils::CheckQName(qName, nsAware)),
-               "Don't pass invalid prefixes to nsDocument::CreateElem, "
+               "Don't pass invalid prefixes to nsIDocument::CreateElem, "
                "check caller.");
 #endif
 
@@ -7974,14 +7972,14 @@ void nsIDocument::OnPageHide(bool aPersisted, EventTarget* aDispatchStartTarget,
     // root document, as this will exit fullscreen in all the root's
     // descendant documents. Note that documents are removed from the
     // doctree by the time OnPageHide() is called, so we must store a
-    // reference to the root (in nsDocument::mFullscreenRoot) since we can't
+    // reference to the root (in nsIDocument::mFullscreenRoot) since we can't
     // just traverse the doctree to get the root.
     nsIDocument::ExitFullscreenInDocTree(this);
 
     // Since the document is removed from the doctree before OnPageHide() is
     // called, ExitFullscreen() can't traverse from the root down to *this*
     // document, so we must manually call CleanupFullscreenState() below too.
-    // Note that CleanupFullscreenState() clears nsDocument::mFullscreenRoot,
+    // Note that CleanupFullscreenState() clears nsIDocument::mFullscreenRoot,
     // so we *must* call it after ExitFullscreen(), not before.
     // OnPageHide() is called in every hidden (i.e. descendant) document,
     // so calling CleanupFullscreenState() here will ensure all hidden
@@ -8157,7 +8155,7 @@ nsresult nsIDocument::CloneDocHelper(nsIDocument* clone) const {
   clone->SetContentTypeInternal(GetContentTypeInternal());
   clone->mSecurityInfo = mSecurityInfo;
 
-  // State from nsDocument
+  // State from nsIDocument
   clone->mType = mType;
   clone->mXMLDeclarationBits = mXMLDeclarationBits;
   clone->mBaseTarget = mBaseTarget;
@@ -8582,18 +8580,15 @@ nsIDocument* nsIDocument::GetTemplateContentsOwner() {
     mTemplateContentsOwner = document;
     NS_ENSURE_TRUE(mTemplateContentsOwner, nullptr);
 
-    nsDocument* doc = static_cast<nsDocument*>(mTemplateContentsOwner.get());
-
     if (!scriptObject) {
       mTemplateContentsOwner->SetScopeObject(GetScopeObject());
     }
 
-    doc->mHasHadScriptHandlingObject = hasHadScriptObject;
+    mTemplateContentsOwner->mHasHadScriptHandlingObject = hasHadScriptObject;
 
-    // Set |doc| as the template contents owner of itself so that
-    // |doc| is the template contents owner of template elements created
-    // by |doc|.
-    doc->mTemplateContentsOwner = doc;
+    // Set |mTemplateContentsOwner| as the template contents owner of itself so
+    // that it is the template contents owner of nested template elements.
+    mTemplateContentsOwner->mTemplateContentsOwner = mTemplateContentsOwner;
   }
 
   return mTemplateContentsOwner;
@@ -8877,24 +8872,22 @@ void nsIDocument::FlushPendingLinkUpdates() {
 
 already_AddRefed<nsIDocument> nsIDocument::CreateStaticClone(
     nsIDocShell* aCloneContainer) {
-  nsDocument* thisAsDoc = static_cast<nsDocument*>(this);
   mCreatingStaticClone = true;
 
   // Make document use different container during cloning.
   RefPtr<nsDocShell> originalShell = mDocumentContainer.get();
   SetContainer(static_cast<nsDocShell*>(aCloneContainer));
   ErrorResult rv;
-  nsCOMPtr<nsINode> clonedNode = thisAsDoc->CloneNode(true, rv);
+  nsCOMPtr<nsINode> clonedNode = this->CloneNode(true, rv);
   SetContainer(originalShell);
 
-  RefPtr<nsDocument> clonedDoc;
+  nsCOMPtr<nsIDocument> clonedDoc;
   if (rv.Failed()) {
     // Don't return yet; we need to reset mCreatingStaticClone
     rv.SuppressException();
   } else {
-    nsCOMPtr<nsIDocument> tmp = do_QueryInterface(clonedNode);
-    if (tmp) {
-      clonedDoc = static_cast<nsDocument*>(tmp.get());
+    clonedDoc = do_QueryInterface(clonedNode);
+    if (clonedDoc) {
       if (IsStaticDocument()) {
         clonedDoc->mOriginalDocument = mOriginalDocument;
       } else {
@@ -9716,7 +9709,7 @@ bool FullscreenRoots::IsEmpty() { return !sInstance; }
 
 // Any fullscreen change waiting for the widget to finish transition
 // is queued here. This is declared static instead of a member of
-// nsDocument because in the majority of time, there would be at most
+// nsIDocument because in the majority of time, there would be at most
 // one document requesting or exiting fullscreen. We shouldn't waste
 // the space to hold for it in every document.
 class PendingFullscreenChangeList {
@@ -10577,7 +10570,7 @@ bool nsIDocument::ApplyFullscreen(UniquePtr<FullscreenRequest> aRequest) {
     }
     nsIDocument* parent = child->GetParentDocument();
     Element* element = parent->FindContentForSubDocument(child);
-    if (static_cast<nsDocument*>(parent)->FullscreenStackPush(element)) {
+    if (parent->FullscreenStackPush(element)) {
       changed.AppendElement(parent);
       child = parent;
     } else {
@@ -10761,18 +10754,17 @@ NS_IMETHODIMP
 PointerLockRequest::Run() {
   nsCOMPtr<Element> e = do_QueryReferent(mElement);
   nsCOMPtr<nsIDocument> doc = do_QueryReferent(mDocument);
-  nsDocument* d = static_cast<nsDocument*>(doc.get());
   const char* error = nullptr;
-  if (!e || !d || !e->GetComposedDoc()) {
+  if (!e || !doc || !e->GetComposedDoc()) {
     error = "PointerLockDeniedNotInDocument";
-  } else if (e->GetComposedDoc() != d) {
+  } else if (e->GetComposedDoc() != doc) {
     error = "PointerLockDeniedMovedDocument";
   }
   if (!error) {
     nsCOMPtr<Element> pointerLockedElement =
         do_QueryReferent(EventStateManager::sPointerLockedElement);
     if (e == pointerLockedElement) {
-      DispatchPointerLockChange(d);
+      DispatchPointerLockChange(doc);
       return NS_OK;
     }
     // Note, we must bypass focus change, so pass true as the last parameter!
@@ -10780,7 +10772,7 @@ PointerLockRequest::Run() {
     // Another element in the same document is requesting pointer lock,
     // just grant it without user input check.
     if (!error && pointerLockedElement) {
-      ChangePointerLockedElement(e, d, pointerLockedElement);
+      ChangePointerLockedElement(e, doc, pointerLockedElement);
       return NS_OK;
     }
   }
@@ -10789,15 +10781,15 @@ PointerLockRequest::Run() {
   if (!error && !mUserInputOrChromeCaller && !doc->GetFullscreenElement()) {
     error = "PointerLockDeniedNotInputDriven";
   }
-  if (!error && !d->SetPointerLock(e, NS_STYLE_CURSOR_NONE)) {
+  if (!error && !doc->SetPointerLock(e, NS_STYLE_CURSOR_NONE)) {
     error = "PointerLockDeniedFailedToLock";
   }
   if (error) {
-    DispatchPointerLockError(d, error);
+    DispatchPointerLockError(doc, error);
     return NS_OK;
   }
 
-  ChangePointerLockedElement(e, d, nullptr);
+  ChangePointerLockedElement(e, doc, nullptr);
   nsContentUtils::DispatchEventOnlyToChrome(
       doc, ToSupports(e), NS_LITERAL_STRING("MozDOMPointerLock:Entered"),
       CanBubble::eYes, Cancelable::eNo, /* DefaultAction */ nullptr);
@@ -10807,7 +10799,7 @@ PointerLockRequest::Run() {
 void nsIDocument::RequestPointerLock(Element* aElement,
                                      CallerType aCallerType) {
   NS_ASSERTION(aElement,
-               "Must pass non-null element to nsDocument::RequestPointerLock");
+               "Must pass non-null element to nsIDocument::RequestPointerLock");
 
   nsCOMPtr<Element> pointerLockedElement =
       do_QueryReferent(EventStateManager::sPointerLockedElement);
@@ -11189,8 +11181,7 @@ nsIDocument* nsIDocument::GetTopLevelContentDocument() {
       return nullptr;
     }
 
-    nsIDocument* candidate = parent->GetParentDocument();
-    parent = static_cast<nsDocument*>(candidate);
+    parent = parent->GetParentDocument();
   } while (parent);
 
   return parent;
@@ -11306,8 +11297,8 @@ bool nsIDocument::InlineScriptAllowedByCSP() {
 static bool ReportExternalResourceUseCounters(nsIDocument* aDocument,
                                               void* aData) {
   const auto reportKind =
-      nsDocument::UseCounterReportKind::eIncludeExternalResources;
-  static_cast<nsDocument*>(aDocument)->ReportUseCounters(reportKind);
+      nsIDocument::UseCounterReportKind::eIncludeExternalResources;
+  aDocument->ReportUseCounters(reportKind);
   return true;
 }
 
@@ -11461,8 +11452,8 @@ void nsIDocument::ScheduleIntersectionObserverNotification() {
   }
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
   nsCOMPtr<nsIRunnable> notification =
-      NewRunnableMethod("nsDocument::NotifyIntersectionObservers", this,
-                        &nsDocument::NotifyIntersectionObservers);
+      NewRunnableMethod("nsIDocument::NotifyIntersectionObservers", this,
+                        &nsIDocument::NotifyIntersectionObservers);
   Dispatch(TaskCategory::Other, notification.forget());
 }
 
@@ -12065,7 +12056,7 @@ FlashClassification nsIDocument::PrincipalFlashClassification() {
 }
 
 /**
- * Helper function for |nsDocument::PrincipalFlashClassification|
+ * Helper function for |nsIDocument::PrincipalFlashClassification|
  *
  * Adds a table name string to a table list (a comma separated string). The
  * table will not be added if the name is an empty string.
@@ -12082,7 +12073,7 @@ static void MaybeAddTableToTableList(const nsACString& aTableNames,
 }
 
 /**
- * Helper function for |nsDocument::PrincipalFlashClassification|
+ * Helper function for |nsIDocument::PrincipalFlashClassification|
  *
  * Takes an array of table names and a comma separated list of table names
  * Returns |true| if any table name in the array matches a table name in the
@@ -12487,7 +12478,7 @@ FlashClassification nsIDocument::DocumentFlashClassification() {
     mFlashClassification = result;
     MOZ_ASSERT(
         result != FlashClassification::Unclassified,
-        "nsDocument::GetPluginClassification should never return Unclassified");
+        "nsIDocument::GetPluginClassification should never return Unclassified");
   }
 
   return mFlashClassification;
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
deleted file mode 100644
index 5aef98593d1e..000000000000
--- a/dom/base/nsDocument.h
+++ /dev/null
@@ -1,35 +0,0 @@
-/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* vim: set ts=8 sts=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/. */
-
-/*
- * Base class for all our document implementations.
- */
-
-#ifndef nsDocument_h___
-#define nsDocument_h___
-
-#include "nsIDocument.h"
-
-#define XML_DECLARATION_BITS_DECLARATION_EXISTS (1 << 0)
-#define XML_DECLARATION_BITS_ENCODING_EXISTS (1 << 1)
-#define XML_DECLARATION_BITS_STANDALONE_EXISTS (1 << 2)
-#define XML_DECLARATION_BITS_STANDALONE_YES (1 << 3)
-
-// Base class for our document implementations.
-class nsDocument : public nsIDocument {
- protected:
-
-  explicit nsDocument(const char* aContentType);
-  virtual ~nsDocument();
-
-  // Don't add stuff here, add to nsIDocument instead.
-
- private:
-  nsDocument(const nsDocument& aOther) = delete;
-  nsDocument& operator=(const nsDocument& aOther) = delete;
-};
-
-#endif /* nsDocument_h___ */
diff --git a/dom/base/nsFocusManager.cpp b/dom/base/nsFocusManager.cpp
index b2e5e9195b20..a9d430adf325 100644
--- a/dom/base/nsFocusManager.cpp
+++ b/dom/base/nsFocusManager.cpp
@@ -13,7 +13,7 @@
 #include "nsGkAtoms.h"
 #include "nsGlobalWindow.h"
 #include "nsContentUtils.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsIContentParent.h"
 #include "nsPIDOMWindow.h"
 #include "nsIDOMChromeWindow.h"
@@ -986,7 +986,7 @@ nsFocusManager::FireDelayedEvents(nsIDocument* aDocument) {
           !aDocument->GetInnerWindow()->IsCurrentInnerWindow()) {
         // If the document was navigated away from or is defunct, don't bother
         // firing events on it. Note the symmetry between this condition and
-        // the similar one in nsDocument.cpp:FireOrClearDelayedEvents.
+        // the similar one in nsIDocument.cpp:FireOrClearDelayedEvents.
         mDelayedBlurFocusEvents.RemoveElementAt(i);
         --i;
       } else if (!aDocument->EventHandlingSuppressed()) {
diff --git a/dom/base/nsGlobalWindowInner.cpp b/dom/base/nsGlobalWindowInner.cpp
index 6ba86bff9224..d6cf2971f1b0 100644
--- a/dom/base/nsGlobalWindowInner.cpp
+++ b/dom/base/nsGlobalWindowInner.cpp
@@ -1999,7 +1999,7 @@ nsresult nsGlobalWindowInner::PostHandleEvent(EventChainPostVisitor& aVisitor) {
   } else if (aVisitor.mEvent->mMessage == eLoad &&
              aVisitor.mEvent->IsTrusted()) {
     // This is page load event since load events don't propagate to |window|.
-    // @see nsDocument::GetEventTargetParent.
+    // @see nsIDocument::GetEventTargetParent.
     mIsDocumentLoaded = true;
 
     mTimeoutManager->OnDocumentLoaded();
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 91313fd094d7..589dc4a43839 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -451,6 +451,12 @@ class nsIDocument : public nsINode,
   template <typename T>
   using NotNull = mozilla::NotNull<T>;
 
+  explicit nsIDocument(const char* aContentType);
+  virtual ~nsIDocument();
+
+  nsIDocument(const nsIDocument&) = delete;
+  nsIDocument& operator=(const nsIDocument&) = delete;
+
  public:
   typedef nsExternalResourceMap::ExternalResourceLoad ExternalResourceLoad;
   typedef mozilla::FullscreenRequest FullscreenRequest;
@@ -478,10 +484,6 @@ class nsIDocument : public nsINode,
     }                                                                         \
   } while (0)
 
-#ifdef MOZILLA_INTERNAL_API
-  nsIDocument();
-#endif
-
   // nsIApplicationCacheContainer
   NS_DECL_NSIAPPLICATIONCACHECONTAINER
 
@@ -624,7 +626,7 @@ class nsIDocument : public nsINode,
 
   /**
    * Signal that the document title may have changed
-   * (see nsDocument::GetTitle).
+   * (see nsIDocument::GetTitle).
    * @param aBoundTitleElement true if an HTML or SVG <title> element
    * has just been bound to the document.
    */
@@ -1902,7 +1904,7 @@ class nsIDocument : public nsINode,
 
   // Notify that a document state has changed.
   // This should only be called by callers whose state is also reflected in the
-  // implementation of nsDocument::GetDocumentState.
+  // implementation of nsIDocument::GetDocumentState.
   void DocumentStatesChanged(mozilla::EventStates aStateMask);
 
   // Observation hooks for style data to propagate notifications
@@ -3624,8 +3626,6 @@ class nsIDocument : public nsINode,
     }
   }
 
-  ~nsIDocument();
-
   // Never ever call this. Only call GetWindow!
   nsPIDOMWindowOuter* GetWindowInternal() const;
 
@@ -3813,7 +3813,7 @@ class nsIDocument : public nsINode,
 
   // This flag is only set in XMLDocument, for e.g. documents used in XBL. We
   // don't want animations to play in such documents, so we need to store the
-  // flag here so that we can check it in nsDocument::GetAnimationController.
+  // flag here so that we can check it in nsIDocument::GetAnimationController.
   bool mLoadedAsInteractiveData : 1;
 
   // If true, whoever is creating the document has gotten it to the
@@ -4108,7 +4108,7 @@ class nsIDocument : public nsINode,
 
   nsCString mContentLanguage;
 
-  // The channel that got passed to nsDocument::StartDocumentLoad(), if any.
+  // The channel that got passed to nsIDocument::StartDocumentLoad(), if any.
   nsCOMPtr<nsIChannel> mChannel;
 
  private:
@@ -4313,6 +4313,8 @@ class nsIDocument : public nsINode,
 
   nsDocHeaderData* mHeaderData;
 
+  // For determining if this is a flash document which should be
+  // blocked based on its principal.
   RefPtr<PrincipalFlashClassifier> mPrincipalFlashClassifier;
   mozilla::dom::FlashClassification mFlashClassification;
   // Do not use this value directly. Call the |IsThirdParty()| method, which
diff --git a/dom/base/nsINode.cpp b/dom/base/nsINode.cpp
index f445faf38357..0295e5bd3a51 100644
--- a/dom/base/nsINode.cpp
+++ b/dom/base/nsINode.cpp
@@ -43,7 +43,7 @@
 #include "nsContentList.h"
 #include "nsContentUtils.h"
 #include "nsCycleCollectionParticipant.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "mozilla/dom/Attr.h"
 #include "nsDOMAttributeMap.h"
 #include "nsDOMCID.h"
@@ -1791,7 +1791,7 @@ void nsINode::RemoveChildNode(nsIContent* aKid, bool aNotify) {
   // nsIDocument::GetRootElement() calls until *after* it has removed aKid from
   // aChildArray. Any calls before then could potentially restore a stale
   // value for our cached root element, per note in
-  // nsDocument::RemoveChildNode().
+  // nsIDocument::RemoveChildNode().
   MOZ_ASSERT(aKid && aKid->GetParentNode() == this, "Bogus aKid");
   MOZ_ASSERT(!IsAttr());
 
diff --git a/dom/base/nsNodeInfoManager.cpp b/dom/base/nsNodeInfoManager.cpp
index 06af241b75d5..ae9641e85deb 100644
--- a/dom/base/nsNodeInfoManager.cpp
+++ b/dom/base/nsNodeInfoManager.cpp
@@ -29,7 +29,7 @@
 #include "nsHashKeys.h"
 #include "nsCCUncollectableMarker.h"
 #include "nsNameSpaceManager.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsWindowSizes.h"
 
 using namespace mozilla;
@@ -85,21 +85,21 @@ NS_IMPL_CYCLE_COLLECTION_UNROOT_NATIVE(nsNodeInfoManager, Release)
 
 NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_BEGIN(nsNodeInfoManager)
   if (tmp->mDocument) {
-    return NS_CYCLE_COLLECTION_PARTICIPANT(nsDocument)
+    return NS_CYCLE_COLLECTION_PARTICIPANT(nsIDocument)
         ->CanSkip(tmp->mDocument, aRemovingAllowed);
   }
 NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_END
 
 NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_IN_CC_BEGIN(nsNodeInfoManager)
   if (tmp->mDocument) {
-    return NS_CYCLE_COLLECTION_PARTICIPANT(nsDocument)
+    return NS_CYCLE_COLLECTION_PARTICIPANT(nsIDocument)
         ->CanSkipInCC(tmp->mDocument);
   }
 NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_IN_CC_END
 
 NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_BEGIN(nsNodeInfoManager)
   if (tmp->mDocument) {
-    return NS_CYCLE_COLLECTION_PARTICIPANT(nsDocument)
+    return NS_CYCLE_COLLECTION_PARTICIPANT(nsIDocument)
         ->CanSkipThis(tmp->mDocument);
   }
 NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_END
diff --git a/dom/base/nsNodeUtils.cpp b/dom/base/nsNodeUtils.cpp
index 84660e17c16a..4152ee51540a 100644
--- a/dom/base/nsNodeUtils.cpp
+++ b/dom/base/nsNodeUtils.cpp
@@ -18,7 +18,7 @@
 #include "PLDHashTable.h"
 #include "nsCOMArray.h"
 #include "nsPIDOMWindow.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #ifdef MOZ_XUL
 #include "nsXULElement.h"
 #endif
diff --git a/dom/base/nsObjectLoadingContent.cpp b/dom/base/nsObjectLoadingContent.cpp
index c880b6e41ae8..205bbeaa2ed4 100644
--- a/dom/base/nsObjectLoadingContent.cpp
+++ b/dom/base/nsObjectLoadingContent.cpp
@@ -918,7 +918,7 @@ nsresult nsObjectLoadingContent::BuildParametersArray() {
 
 void nsObjectLoadingContent::NotifyOwnerDocumentActivityChanged() {
   // XXX(johns): We cannot touch plugins or run arbitrary script from this call,
-  //             as nsDocument is in a non-reentrant state.
+  //             as nsIDocument is in a non-reentrant state.
 
   // If we have a plugin we want to queue an event to stop it unless we are
   // moved into an active document before returning to the event loop.
diff --git a/dom/base/nsObjectLoadingContent.h b/dom/base/nsObjectLoadingContent.h
index 5f5c3a5b0a48..86af042fe9e5 100644
--- a/dom/base/nsObjectLoadingContent.h
+++ b/dom/base/nsObjectLoadingContent.h
@@ -155,7 +155,7 @@ class nsObjectLoadingContent : public nsImageLoadingContent,
 
   /**
    * Notify this class the document state has changed
-   * Called by nsDocument so we may suspend plugins in inactive documents)
+   * Called by nsIDocument so we may suspend plugins in inactive documents)
    */
   void NotifyOwnerDocumentActivityChanged();
 
diff --git a/dom/base/nsTextNode.cpp b/dom/base/nsTextNode.cpp
index de1f10327a7c..72def9f81b12 100644
--- a/dom/base/nsTextNode.cpp
+++ b/dom/base/nsTextNode.cpp
@@ -20,7 +20,7 @@
 #ifdef DEBUG
 #include "nsRange.h"
 #endif
-#include "nsDocument.h"
+#include "nsIDocument.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
diff --git a/dom/console/Console.cpp b/dom/console/Console.cpp
index 1caa8b1ab80a..3ae2a69732f8 100644
--- a/dom/console/Console.cpp
+++ b/dom/console/Console.cpp
@@ -28,7 +28,7 @@
 #include "mozilla/Maybe.h"
 #include "mozilla/StaticPrefs.h"
 #include "nsCycleCollectionParticipant.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsDOMNavigationTiming.h"
 #include "nsGlobalWindow.h"
 #include "nsJSUtils.h"
diff --git a/dom/html/HTMLImageElement.cpp b/dom/html/HTMLImageElement.cpp
index 7ae60b2b7757..8288edc4e6e2 100644
--- a/dom/html/HTMLImageElement.cpp
+++ b/dom/html/HTMLImageElement.cpp
@@ -11,7 +11,7 @@
 #include "nsPresContext.h"
 #include "nsMappedAttributes.h"
 #include "nsSize.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsIDocument.h"
 #include "nsImageFrame.h"
 #include "nsIScriptContext.h"
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 13f2a2f99313..210ba0a86e63 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -51,7 +51,7 @@
 #include "nsIServiceManager.h"
 #include "nsError.h"
 #include "nsIEditor.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsAttrValueOrString.h"
 #include "nsDateTimeControlFrame.h"
 
diff --git a/dom/html/HTMLSlotElement.cpp b/dom/html/HTMLSlotElement.cpp
index 1124b9834307..d201da8d4d62 100644
--- a/dom/html/HTMLSlotElement.cpp
+++ b/dom/html/HTMLSlotElement.cpp
@@ -10,7 +10,7 @@
 #include "mozilla/dom/HTMLUnknownElement.h"
 #include "mozilla/dom/ShadowRoot.h"
 #include "nsGkAtoms.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 
 nsGenericHTMLElement* NS_NewHTMLSlotElement(
     already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
diff --git a/dom/html/HTMLUnknownElement.cpp b/dom/html/HTMLUnknownElement.cpp
index 055de1f86b74..e85f822ec8ae 100644
--- a/dom/html/HTMLUnknownElement.cpp
+++ b/dom/html/HTMLUnknownElement.cpp
@@ -4,7 +4,7 @@
  * 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/. */
 
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "mozilla/dom/HTMLUnknownElement.h"
 #include "mozilla/dom/HTMLElementBinding.h"
 #include "jsapi.h"
diff --git a/dom/html/MediaDocument.cpp b/dom/html/MediaDocument.cpp
index d421ad69633c..ed24e59a720c 100644
--- a/dom/html/MediaDocument.cpp
+++ b/dom/html/MediaDocument.cpp
@@ -143,7 +143,7 @@ nsresult MediaDocument::StartDocumentLoad(const char* aCommand,
                                           nsISupports* aContainer,
                                           nsIStreamListener** aDocListener,
                                           bool aReset, nsIContentSink* aSink) {
-  nsresult rv = nsDocument::StartDocumentLoad(
+  nsresult rv = nsIDocument::StartDocumentLoad(
       aCommand, aChannel, aLoadGroup, aContainer, aDocListener, aReset, aSink);
   if (NS_FAILED(rv)) {
     return rv;
@@ -274,7 +274,7 @@ void MediaDocument::GetFileName(nsAString& aResult, nsIChannel* aChannel) {
   nsAutoCString docCharset;
   // Now that the charset is set in |StartDocumentLoad| to the charset of
   // the document viewer instead of a bogus value ("windows-1252" set in
-  // |nsDocument|'s ctor), the priority is given to the current charset.
+  // |nsIDocument|'s ctor), the priority is given to the current charset.
   // This is necessary to deal with a media document being opened in a new
   // window or a new tab.
   if (mCharacterSetSource != kCharsetUninitialized) {
diff --git a/dom/html/nsHTMLDocument.cpp b/dom/html/nsHTMLDocument.cpp
index 07d430b433ed..1c839aa380e4 100644
--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -165,7 +165,7 @@ nsresult NS_NewHTMLDocument(nsIDocument** aInstancePtrResult,
 }
 
 nsHTMLDocument::nsHTMLDocument()
-    : nsDocument("text/html"),
+    : nsIDocument("text/html"),
       mContentListHolder(nullptr),
       mNumForms(0),
       mWriteLevel(0),
@@ -184,10 +184,10 @@ nsHTMLDocument::nsHTMLDocument()
 
 nsHTMLDocument::~nsHTMLDocument() {}
 
-NS_IMPL_CYCLE_COLLECTION_INHERITED(nsHTMLDocument, nsDocument, mAll,
+NS_IMPL_CYCLE_COLLECTION_INHERITED(nsHTMLDocument, nsIDocument, mAll,
                                    mWyciwygChannel, mMidasCommandManager)
 
-NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(nsHTMLDocument, nsDocument,
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(nsHTMLDocument, nsIDocument,
                                              nsIHTMLDocument)
 
 JSObject* nsHTMLDocument::WrapNode(JSContext* aCx,
@@ -196,7 +196,7 @@ JSObject* nsHTMLDocument::WrapNode(JSContext* aCx,
 }
 
 nsresult nsHTMLDocument::Init() {
-  nsresult rv = nsDocument::Init();
+  nsresult rv = nsIDocument::Init();
   NS_ENSURE_SUCCESS(rv, rv);
 
   // Now reset the compatibility mode of the CSSLoader
@@ -207,7 +207,7 @@ nsresult nsHTMLDocument::Init() {
 }
 
 void nsHTMLDocument::Reset(nsIChannel* aChannel, nsILoadGroup* aLoadGroup) {
-  nsDocument::Reset(aChannel, aLoadGroup);
+  nsIDocument::Reset(aChannel, aLoadGroup);
 
   if (aChannel) {
     aChannel->GetLoadFlags(&mLoadFlags);
@@ -218,7 +218,7 @@ void nsHTMLDocument::ResetToURI(nsIURI* aURI, nsILoadGroup* aLoadGroup,
                                 nsIPrincipal* aPrincipal) {
   mLoadFlags = nsIRequest::LOAD_NORMAL;
 
-  nsDocument::ResetToURI(aURI, aLoadGroup, aPrincipal);
+  nsIDocument::ResetToURI(aURI, aLoadGroup, aPrincipal);
 
   mImages = nullptr;
   mApplets = nullptr;
@@ -443,7 +443,7 @@ void nsHTMLDocument::TryFallback(int32_t& aCharsetSource,
 
 void nsHTMLDocument::SetDocumentCharacterSet(
     NotNull<const Encoding*> aEncoding) {
-  nsDocument::SetDocumentCharacterSet(aEncoding);
+  nsIDocument::SetDocumentCharacterSet(aEncoding);
   // Make sure to stash this charset on our channel as needed if it's a wyciwyg
   // channel.
   nsCOMPtr<nsIWyciwygChannel> wyciwygChannel = do_QueryInterface(mChannel);
@@ -526,7 +526,7 @@ nsresult nsHTMLDocument::StartDocumentLoad(const char* aCommand,
 
   CSSLoader()->SetCompatibilityMode(mCompatMode);
 
-  nsresult rv = nsDocument::StartDocumentLoad(aCommand, aChannel, aLoadGroup,
+  nsresult rv = nsIDocument::StartDocumentLoad(aCommand, aChannel, aLoadGroup,
                                               aContainer, aDocListener, aReset);
   if (NS_FAILED(rv)) {
     return rv;
@@ -771,7 +771,7 @@ void nsHTMLDocument::StopDocumentLoad() {
                "nsHTMLDocument::StopDocumentLoad(): "
                "nsIWyciwygChannel could not be removed!");
 
-  nsDocument::StopDocumentLoad();
+  nsIDocument::StopDocumentLoad();
   UnblockOnload(false);
 }
 
@@ -785,14 +785,14 @@ void nsHTMLDocument::BeginLoad() {
     TurnEditingOff();
     EditingStateChanged();
   }
-  nsDocument::BeginLoad();
+  nsIDocument::BeginLoad();
 }
 
 void nsHTMLDocument::EndLoad() {
   bool turnOnEditing =
       mParser && (HasFlag(NODE_IS_EDITABLE) || mContentEditableCount > 0);
-  // Note: nsDocument::EndLoad nulls out mParser.
-  nsDocument::EndLoad();
+  // Note: nsIDocument::EndLoad nulls out mParser.
+  nsIDocument::EndLoad();
   if (turnOnEditing) {
     EditingStateChanged();
   }
@@ -816,7 +816,7 @@ nsIContent* nsHTMLDocument::GetUnfocusedKeyEventTarget() {
   if (nsGenericHTMLElement* body = GetBody()) {
     return body;
   }
-  return nsDocument::GetUnfocusedKeyEventTarget();
+  return nsIDocument::GetUnfocusedKeyEventTarget();
 }
 
 already_AddRefed<nsIURI> nsHTMLDocument::GetDomainURI() {
@@ -2022,7 +2022,7 @@ void nsHTMLDocument::MaybeEditingStateChanged() {
 void nsHTMLDocument::EndUpdate() {
   const bool reset = !mPendingMaybeEditingStateChanged;
   mPendingMaybeEditingStateChanged = true;
-  nsDocument::EndUpdate();
+  nsIDocument::EndUpdate();
   if (reset) {
     mPendingMaybeEditingStateChanged = false;
   }
@@ -3100,12 +3100,12 @@ bool nsHTMLDocument::IsEditingOnAfterFlush() {
 
 void nsHTMLDocument::RemovedFromDocShell() {
   mEditingState = eOff;
-  nsDocument::RemovedFromDocShell();
+  nsIDocument::RemovedFromDocShell();
 }
 
 /* virtual */ void nsHTMLDocument::DocAddSizeOfExcludingThis(
     nsWindowSizes& aWindowSizes) const {
-  nsDocument::DocAddSizeOfExcludingThis(aWindowSizes);
+  nsIDocument::DocAddSizeOfExcludingThis(aWindowSizes);
 
   // Measurement of the following members may be added later if DMD finds it is
   // worthwhile:
diff --git a/dom/html/nsHTMLDocument.h b/dom/html/nsHTMLDocument.h
index f65ed0037b91..2d706f179f76 100644
--- a/dom/html/nsHTMLDocument.h
+++ b/dom/html/nsHTMLDocument.h
@@ -8,7 +8,7 @@
 
 #include "mozilla/Attributes.h"
 #include "nsContentList.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsIHTMLDocument.h"
 #include "nsIHTMLCollection.h"
 #include "nsIScriptElement.h"
@@ -33,18 +33,18 @@ class HTMLAllCollection;
 }  // namespace dom
 }  // namespace mozilla
 
-class nsHTMLDocument : public nsDocument, public nsIHTMLDocument {
+class nsHTMLDocument : public nsIDocument, public nsIHTMLDocument {
   typedef mozilla::net::ReferrerPolicy ReferrerPolicy;
 
  public:
-  using nsDocument::GetPlugins;
-  using nsDocument::SetDocumentURI;
+  using nsIDocument::GetPlugins;
+  using nsIDocument::SetDocumentURI;
 
   nsHTMLDocument();
   virtual nsresult Init() override;
 
   NS_DECL_ISUPPORTS_INHERITED
-  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(nsHTMLDocument, nsDocument)
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(nsHTMLDocument, nsIDocument)
 
   // nsIDocument
   virtual void Reset(nsIChannel* aChannel, nsILoadGroup* aLoadGroup) override;
diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index 28a8ee8a8e47..8acfcf5951d6 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -977,7 +977,7 @@ nsresult ContentChild::ProvideWindowCommon(
 
     // Set the opener window for this window before we start loading the
     // document inside of it. We have to do this before loading the remote
-    // scripts, because they can poke at the document and cause the nsDocument
+    // scripts, because they can poke at the document and cause the nsIDocument
     // to be created before the openerwindow
     nsCOMPtr<mozIDOMWindowProxy> windowProxy =
         do_GetInterface(newChild->WebNavigation());
diff --git a/dom/smil/SMILAnimationController.h b/dom/smil/SMILAnimationController.h
index 3d27bca85368..7627c4410ed5 100644
--- a/dom/smil/SMILAnimationController.h
+++ b/dom/smil/SMILAnimationController.h
@@ -33,7 +33,7 @@ class SVGAnimationElement;
 //
 // The animation controller maintains the animation timer and determines the
 // sample times and sample rate for all SMIL animations in a document. There is
-// at most one animation controller per nsDocument so that frame-rate tuning can
+// at most one animation controller per document so that frame-rate tuning can
 // be performed at a document-level.
 //
 // The animation controller can contain many child time containers (timed
diff --git a/dom/smil/nsSMILCompositorTable.h b/dom/smil/nsSMILCompositorTable.h
index c8b8bf445509..0e0ecc5e0598 100644
--- a/dom/smil/nsSMILCompositorTable.h
+++ b/dom/smil/nsSMILCompositorTable.h
@@ -13,7 +13,7 @@
 // nsSMILCompositorTable : A hashmap of nsSMILCompositors
 //
 // This is just a forward-declaration because it is included in
-// SMILAnimationController which is used in nsDocument. We don't want to
+// SMILAnimationController which is used in nsIDocument. We don't want to
 // expose all of nsSMILCompositor or otherwise any changes to it will mean the
 // whole world will need to be rebuilt.
 
diff --git a/dom/webidl/Animatable.webidl b/dom/webidl/Animatable.webidl
index 664ec8ea282a..17ffd8d0b47f 100644
--- a/dom/webidl/Animatable.webidl
+++ b/dom/webidl/Animatable.webidl
@@ -23,6 +23,6 @@ interface Animatable {
   [Throws]
   Animation animate(object? keyframes,
                     optional UnrestrictedDoubleOrKeyframeAnimationOptions options);
-  [Func="nsDocument::IsWebAnimationsGetAnimationsEnabled"]
+  [Func="nsIDocument::IsWebAnimationsGetAnimationsEnabled"]
   sequence<Animation> getAnimations(optional AnimationFilter filter);
 };
diff --git a/dom/webidl/Animation.webidl b/dom/webidl/Animation.webidl
index 54e586b01f98..4eadc85f590c 100644
--- a/dom/webidl/Animation.webidl
+++ b/dom/webidl/Animation.webidl
@@ -16,9 +16,9 @@ enum AnimationPlayState { "idle", "running", "paused", "finished" };
               optional AnimationTimeline? timeline)]
 interface Animation : EventTarget {
   attribute DOMString id;
-  [Func="nsDocument::IsWebAnimationsEnabled", Pure]
+  [Func="nsIDocument::IsWebAnimationsEnabled", Pure]
   attribute AnimationEffect? effect;
-  [Func="nsDocument::AreWebAnimationsTimelinesEnabled"]
+  [Func="nsIDocument::AreWebAnimationsTimelinesEnabled"]
   attribute AnimationTimeline? timeline;
   [BinaryName="startTimeAsDouble"]
   attribute double? startTime;
@@ -30,9 +30,9 @@ interface Animation : EventTarget {
   readonly attribute AnimationPlayState playState;
   [BinaryName="pendingFromJS"]
   readonly attribute boolean            pending;
-  [Func="nsDocument::IsWebAnimationsEnabled", Throws]
+  [Func="nsIDocument::IsWebAnimationsEnabled", Throws]
   readonly attribute Promise<Animation> ready;
-  [Func="nsDocument::IsWebAnimationsEnabled", Throws]
+  [Func="nsIDocument::IsWebAnimationsEnabled", Throws]
   readonly attribute Promise<Animation> finished;
            attribute EventHandler       onfinish;
            attribute EventHandler       oncancel;
diff --git a/dom/webidl/AnimationEffect.webidl b/dom/webidl/AnimationEffect.webidl
index 81a04b2193f2..ae37ccf302a9 100644
--- a/dom/webidl/AnimationEffect.webidl
+++ b/dom/webidl/AnimationEffect.webidl
@@ -55,7 +55,7 @@ dictionary ComputedEffectTiming : EffectTiming {
   unrestricted double?  currentIteration = null;
 };
 
-[Func="nsDocument::IsWebAnimationsEnabled"]
+[Func="nsIDocument::IsWebAnimationsEnabled"]
 interface AnimationEffect {
   EffectTiming getTiming();
   [BinaryName="getComputedTimingAsDict"]
diff --git a/dom/webidl/AnimationPlaybackEvent.webidl b/dom/webidl/AnimationPlaybackEvent.webidl
index db8136eae66e..329006037677 100644
--- a/dom/webidl/AnimationPlaybackEvent.webidl
+++ b/dom/webidl/AnimationPlaybackEvent.webidl
@@ -10,7 +10,7 @@
  * liability, trademark and document use rules apply.
  */
 
-[Func="nsDocument::IsWebAnimationsEnabled",
+[Func="nsIDocument::IsWebAnimationsEnabled",
  Constructor(DOMString type,
              optional AnimationPlaybackEventInit eventInitDict)]
 interface AnimationPlaybackEvent : Event {
diff --git a/dom/webidl/AnimationTimeline.webidl b/dom/webidl/AnimationTimeline.webidl
index 7a1bc0cb713c..d00aea7caca1 100644
--- a/dom/webidl/AnimationTimeline.webidl
+++ b/dom/webidl/AnimationTimeline.webidl
@@ -10,7 +10,7 @@
  * liability, trademark and document use rules apply.
  */
 
-[Func="nsDocument::AreWebAnimationsTimelinesEnabled"]
+[Func="nsIDocument::AreWebAnimationsTimelinesEnabled"]
 interface AnimationTimeline {
   [BinaryName="currentTimeAsDouble"]
   readonly attribute double? currentTime;
diff --git a/dom/webidl/CSSAnimation.webidl b/dom/webidl/CSSAnimation.webidl
index 63abf6c7a39a..ca5fc7d001c1 100644
--- a/dom/webidl/CSSAnimation.webidl
+++ b/dom/webidl/CSSAnimation.webidl
@@ -10,7 +10,7 @@
  * liability, trademark and document use rules apply.
  */
 
-[Func="nsDocument::IsWebAnimationsGetAnimationsEnabled",
+[Func="nsIDocument::IsWebAnimationsGetAnimationsEnabled",
  HeaderFile="nsAnimationManager.h"]
 interface CSSAnimation : Animation {
   [Constant] readonly attribute DOMString animationName;
diff --git a/dom/webidl/CSSPseudoElement.webidl b/dom/webidl/CSSPseudoElement.webidl
index 903cf3dd44f3..8659872c6b95 100644
--- a/dom/webidl/CSSPseudoElement.webidl
+++ b/dom/webidl/CSSPseudoElement.webidl
@@ -15,7 +15,7 @@
 // this interface.
 // What we implement here is a minimal subset of the two definitions which we
 // ship behind a pref until the specification issues have been resolved.
-[Func="nsDocument::IsWebAnimationsGetAnimationsEnabled"]
+[Func="nsIDocument::IsWebAnimationsGetAnimationsEnabled"]
 interface CSSPseudoElement {
   readonly attribute DOMString type;
   readonly attribute Element parentElement;
diff --git a/dom/webidl/CSSTransition.webidl b/dom/webidl/CSSTransition.webidl
index 25a8ca92d9ed..2d477944423e 100644
--- a/dom/webidl/CSSTransition.webidl
+++ b/dom/webidl/CSSTransition.webidl
@@ -10,7 +10,7 @@
  * liability, trademark and document use rules apply.
  */
 
-[Func="nsDocument::IsWebAnimationsGetAnimationsEnabled",
+[Func="nsIDocument::IsWebAnimationsGetAnimationsEnabled",
  HeaderFile="nsTransitionManager.h"]
 interface CSSTransition : Animation {
   [Constant] readonly attribute DOMString transitionProperty;
diff --git a/dom/webidl/Document.webidl b/dom/webidl/Document.webidl
index 7e9015185172..d716b0ff0fe2 100644
--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -261,24 +261,24 @@ partial interface Document {
 partial interface Document {
   // Note: Per spec the 'S' in these two is lowercase, but the "Moz"
   // versions have it uppercase.
-  [LenientSetter, Unscopable, Func="nsDocument::IsUnprefixedFullscreenEnabled"]
+  [LenientSetter, Unscopable, Func="nsIDocument::IsUnprefixedFullscreenEnabled"]
   readonly attribute boolean fullscreen;
   [BinaryName="fullscreen"]
   readonly attribute boolean mozFullScreen;
-  [LenientSetter, Func="nsDocument::IsUnprefixedFullscreenEnabled", NeedsCallerType]
+  [LenientSetter, Func="nsIDocument::IsUnprefixedFullscreenEnabled", NeedsCallerType]
   readonly attribute boolean fullscreenEnabled;
   [BinaryName="fullscreenEnabled", NeedsCallerType]
   readonly attribute boolean mozFullScreenEnabled;
 
-  [Throws, Func="nsDocument::IsUnprefixedFullscreenEnabled"]
+  [Throws, Func="nsIDocument::IsUnprefixedFullscreenEnabled"]
   Promise<void> exitFullscreen();
   [Throws, BinaryName="exitFullscreen"]
   Promise<void> mozCancelFullScreen();
 
   // Events handlers
-  [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
+  [Func="nsIDocument::IsUnprefixedFullscreenEnabled"]
   attribute EventHandler onfullscreenchange;
-  [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
+  [Func="nsIDocument::IsUnprefixedFullscreenEnabled"]
   attribute EventHandler onfullscreenerror;
 };
 
@@ -329,9 +329,9 @@ partial interface Document {
 
 // https://drafts.csswg.org/web-animations/#extensions-to-the-document-interface
 partial interface Document {
-  [Func="nsDocument::AreWebAnimationsTimelinesEnabled"]
+  [Func="nsIDocument::AreWebAnimationsTimelinesEnabled"]
   readonly attribute DocumentTimeline timeline;
-  [Func="nsDocument::IsWebAnimationsGetAnimationsEnabled"]
+  [Func="nsIDocument::IsWebAnimationsGetAnimationsEnabled"]
   sequence<Animation> getAnimations();
 };
 
@@ -528,7 +528,7 @@ partial interface Document {
 };
 
 partial interface Document {
-  [Func="nsDocument::DocumentSupportsL10n"] readonly attribute DocumentL10n? l10n;
+  [Func="nsIDocument::DocumentSupportsL10n"] readonly attribute DocumentL10n? l10n;
 };
 
 Document implements XPathEvaluator;
diff --git a/dom/webidl/DocumentTimeline.webidl b/dom/webidl/DocumentTimeline.webidl
index 4837bd2ec79c..c7b8dd25d7b4 100644
--- a/dom/webidl/DocumentTimeline.webidl
+++ b/dom/webidl/DocumentTimeline.webidl
@@ -14,7 +14,7 @@ dictionary DocumentTimelineOptions {
   DOMHighResTimeStamp originTime = 0;
 };
 
-[Func="nsDocument::AreWebAnimationsTimelinesEnabled",
+[Func="nsIDocument::AreWebAnimationsTimelinesEnabled",
  Constructor (optional DocumentTimelineOptions options)]
 interface DocumentTimeline : AnimationTimeline {
 };
diff --git a/dom/webidl/Element.webidl b/dom/webidl/Element.webidl
index 95ef2f0bf86d..2523f316417d 100644
--- a/dom/webidl/Element.webidl
+++ b/dom/webidl/Element.webidl
@@ -240,7 +240,7 @@ partial interface Element {
   [BinaryName="shadowRootByMode"]
   readonly attribute ShadowRoot? shadowRoot;
 
-  [Func="nsDocument::IsCallerChromeOrAddon", BinaryName="shadowRoot"]
+  [Func="nsIDocument::IsCallerChromeOrAddon", BinaryName="shadowRoot"]
   readonly attribute ShadowRoot? openOrClosedShadowRoot;
 
   [BinaryName="assignedSlotByMode"]
@@ -261,15 +261,15 @@ Element implements GeometryUtils;
 
 // https://fullscreen.spec.whatwg.org/#api
 partial interface Element {
-  [Throws, Func="nsDocument::IsUnprefixedFullscreenEnabled", NeedsCallerType]
+  [Throws, Func="nsIDocument::IsUnprefixedFullscreenEnabled", NeedsCallerType]
   Promise<void> requestFullscreen();
   [Throws, BinaryName="requestFullscreen", NeedsCallerType, Deprecated="MozRequestFullScreenDeprecatedPrefix"]
   Promise<void> mozRequestFullScreen();
 
   // Events handlers
-  [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
+  [Func="nsIDocument::IsUnprefixedFullscreenEnabled"]
   attribute EventHandler onfullscreenchange;
-  [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
+  [Func="nsIDocument::IsUnprefixedFullscreenEnabled"]
   attribute EventHandler onfullscreenerror;
 };
 
diff --git a/dom/webidl/KeyframeEffect.webidl b/dom/webidl/KeyframeEffect.webidl
index 4d7e1ba632f9..47e09caccb7f 100644
--- a/dom/webidl/KeyframeEffect.webidl
+++ b/dom/webidl/KeyframeEffect.webidl
@@ -22,7 +22,7 @@ dictionary KeyframeEffectOptions : EffectTiming {
 
 // KeyframeEffect should run in the caller's compartment to do custom
 // processing on the `keyframes` object.
-[Func="nsDocument::IsWebAnimationsEnabled",
+[Func="nsIDocument::IsWebAnimationsEnabled",
  RunConstructorInCallerCompartment,
  Constructor ((Element or CSSPseudoElement)? target,
               object? keyframes,
diff --git a/dom/xml/XMLDocument.cpp b/dom/xml/XMLDocument.cpp
index c5fe4bf622fa..d191c35b31d2 100644
--- a/dom/xml/XMLDocument.cpp
+++ b/dom/xml/XMLDocument.cpp
@@ -125,12 +125,11 @@ nsresult NS_NewDOMDocument(nsIDocument** aInstancePtrResult,
     htmlDoc->SetCompatibilityMode(eCompatibility_FullStandards);
     htmlDoc->SetIsXHTML(isXHTML);
   }
-  nsDocument* doc = static_cast<nsDocument*>(d.get());
-  doc->SetLoadedAsData(aLoadedAsData);
-  doc->nsDocument::SetDocumentURI(aDocumentURI);
+  d->SetLoadedAsData(aLoadedAsData);
+  d->SetDocumentURI(aDocumentURI);
   // Must set the principal first, since SetBaseURI checks it.
-  doc->SetPrincipal(aPrincipal);
-  doc->SetBaseURI(aBaseURI);
+  d->SetPrincipal(aPrincipal);
+  d->SetBaseURI(aBaseURI);
 
   // We need to set the script handling object after we set the principal such
   // that the doc group is assigned correctly.
@@ -142,7 +141,7 @@ nsresult NS_NewDOMDocument(nsIDocument** aInstancePtrResult,
 
   // XMLDocuments and documents "created in memory" get to be UTF-8 by default,
   // unlike the legacy HTML mess
-  doc->SetDocumentCharacterSet(UTF_8_ENCODING);
+  d->SetDocumentCharacterSet(UTF_8_ENCODING);
 
   if (aDoctype) {
     ErrorResult result;
@@ -162,7 +161,7 @@ nsresult NS_NewDOMDocument(nsIDocument** aInstancePtrResult,
     options.SetAsString();
 
     nsCOMPtr<Element> root =
-        doc->CreateElementNS(aNamespaceURI, aQualifiedName, options, result);
+        d->CreateElementNS(aNamespaceURI, aQualifiedName, options, result);
     if (NS_WARN_IF(result.Failed())) {
       return result.StealNSResult();
     }
@@ -209,14 +208,13 @@ nsresult NS_NewXBLDocument(nsIDocument** aInstancePtrResult,
       aPrincipal, false, nullptr, DocumentFlavorLegacyGuess);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  nsIDocument* idoc = *aInstancePtrResult;
+  nsIDocument* doc = *aInstancePtrResult;
 
   // XBL documents must allow XUL and XBL elements in them but the usual check
   // only checks if the document is loaded in the system principal which is
   // sometimes not the case.
-  idoc->ForceEnableXULXBL();
+  doc->ForceEnableXULXBL();
 
-  nsDocument* doc = static_cast<nsDocument*>(idoc);
   doc->SetLoadedAsInteractiveData(true);
   doc->SetReadyStateInternal(nsIDocument::READYSTATE_COMPLETE);
 
@@ -227,7 +225,7 @@ namespace mozilla {
 namespace dom {
 
 XMLDocument::XMLDocument(const char* aContentType)
-    : nsDocument(aContentType),
+    : nsIDocument(aContentType),
       mChannelIsPending(false),
       mAsync(true),
       mLoopingForSyncLoad(false),
@@ -243,14 +241,14 @@ XMLDocument::~XMLDocument() {
 }
 
 nsresult XMLDocument::Init() {
-  nsresult rv = nsDocument::Init();
+  nsresult rv = nsIDocument::Init();
   NS_ENSURE_SUCCESS(rv, rv);
 
   return rv;
 }
 
 void XMLDocument::Reset(nsIChannel* aChannel, nsILoadGroup* aLoadGroup) {
-  nsDocument::Reset(aChannel, aLoadGroup);
+  nsIDocument::Reset(aChannel, aLoadGroup);
 }
 
 void XMLDocument::ResetToURI(nsIURI* aURI, nsILoadGroup* aLoadGroup,
@@ -261,7 +259,7 @@ void XMLDocument::ResetToURI(nsIURI* aURI, nsILoadGroup* aLoadGroup,
     mChannelIsPending = false;
   }
 
-  nsDocument::ResetToURI(aURI, aLoadGroup, aPrincipal);
+  nsIDocument::ResetToURI(aURI, aLoadGroup, aPrincipal);
 }
 
 bool XMLDocument::Load(const nsAString& aUrl, CallerType aCallerType,
@@ -479,7 +477,7 @@ nsresult XMLDocument::StartDocumentLoad(const char* aCommand,
                                         nsISupports* aContainer,
                                         nsIStreamListener** aDocListener,
                                         bool aReset, nsIContentSink* aSink) {
-  nsresult rv = nsDocument::StartDocumentLoad(
+  nsresult rv = nsIDocument::StartDocumentLoad(
       aCommand, aChannel, aLoadGroup, aContainer, aDocListener, aReset, aSink);
   if (NS_FAILED(rv)) return rv;
 
@@ -537,10 +535,10 @@ void XMLDocument::EndLoad() {
   mLoopingForSyncLoad = false;
 
   mSynchronousDOMContentLoaded = (mLoadedAsData || mLoadedAsInteractiveData);
-  nsDocument::EndLoad();
+  nsIDocument::EndLoad();
   if (mSynchronousDOMContentLoaded) {
     mSynchronousDOMContentLoaded = false;
-    nsDocument::SetReadyStateInternal(nsIDocument::READYSTATE_COMPLETE);
+    nsIDocument::SetReadyStateInternal(nsIDocument::READYSTATE_COMPLETE);
     // Generate a document load event for the case when an XML
     // document was loaded as pure data without any presentation
     // attached to it.
@@ -551,7 +549,7 @@ void XMLDocument::EndLoad() {
 
 /* virtual */ void XMLDocument::DocAddSizeOfExcludingThis(
     nsWindowSizes& aWindowSizes) const {
-  nsDocument::DocAddSizeOfExcludingThis(aWindowSizes);
+  nsIDocument::DocAddSizeOfExcludingThis(aWindowSizes);
 }
 
 // nsIDocument interface
diff --git a/dom/xml/XMLDocument.h b/dom/xml/XMLDocument.h
index 1381c6cdd8bc..5f4042350cea 100644
--- a/dom/xml/XMLDocument.h
+++ b/dom/xml/XMLDocument.h
@@ -9,7 +9,7 @@
 
 #include "mozilla/Attributes.h"
 #include "mozilla/dom/BindingDeclarations.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsIScriptContext.h"
 
 class nsIURI;
@@ -18,11 +18,11 @@ class nsIChannel;
 namespace mozilla {
 namespace dom {
 
-class XMLDocument : public nsDocument {
+class XMLDocument : public nsIDocument {
  public:
   explicit XMLDocument(const char* aContentType = "application/xml");
 
-  NS_INLINE_DECL_REFCOUNTING_INHERITED(XMLDocument, nsDocument)
+  NS_INLINE_DECL_REFCOUNTING_INHERITED(XMLDocument, nsIDocument)
 
   virtual void Reset(nsIChannel* aChannel, nsILoadGroup* aLoadGroup) override;
   virtual void ResetToURI(nsIURI* aURI, nsILoadGroup* aLoadGroup,
@@ -58,7 +58,7 @@ class XMLDocument : public nsDocument {
 
   // .location is [Unforgeable], so we have to make it clear that the
   // nsIDocument version applies to us (it's shadowed by the XPCOM thing on
-  // nsDocument).
+  // nsIDocument).
   using nsIDocument::GetLocation;
 
  protected:
diff --git a/dom/xml/nsXMLContentSink.cpp b/dom/xml/nsXMLContentSink.cpp
index 2e9cbd18a3db..b2f24b01eb6a 100644
--- a/dom/xml/nsXMLContentSink.cpp
+++ b/dom/xml/nsXMLContentSink.cpp
@@ -241,7 +241,7 @@ NS_IMETHODIMP
 nsXMLContentSink::DidBuildModel(bool aTerminated) {
   if (!mParser) {
     // If mParser is null, this parse has already been terminated and must
-    // not been terminated again. However, nsDocument may still think that
+    // not been terminated again. However, nsIDocument may still think that
     // the parse has not been terminated and call back into here in the case
     // where the XML parser has finished but the XSLT transform associated
     // with the document has not.
diff --git a/dom/xul/XULDocument.cpp b/dom/xul/XULDocument.cpp
index 005fc13f252f..0a2eff4c54be 100644
--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -140,7 +140,7 @@ XULDocument::XULDocument(void)
       mOffThreadCompileStringBuf(nullptr),
       mOffThreadCompileStringLength(0),
       mInitialLayoutComplete(false) {
-  // Override the default in nsDocument
+  // Override the default in nsIDocument
   mCharacterSet = UTF_8_ENCODING;
 
   mDefaultElementType = kNameSpaceID_XUL;
@@ -254,7 +254,7 @@ nsresult XULDocument::StartDocumentLoad(const char* aCommand,
       }
     }
   }
-  // NOTE: If this ever starts calling nsDocument::StartDocumentLoad
+  // NOTE: If this ever starts calling nsIDocument::StartDocumentLoad
   // we'll possibly need to reset our content type afterwards.
   mStillWalking = true;
   mMayStartLayout = false;
diff --git a/editor/composer/moz.build b/editor/composer/moz.build
index e4d92a0308bb..f6b07087d6d9 100644
--- a/editor/composer/moz.build
+++ b/editor/composer/moz.build
@@ -27,7 +27,7 @@ EXPORTS.mozilla += [
     'ComposerCommandsUpdater.h',
 ]
 
-# Needed because we include HTMLEditor.h which indirectly includes nsDocument.h
+# Needed because we include HTMLEditor.h which indirectly includes nsIDocument.h
 LOCAL_INCLUDES += [
     '/dom/base',
     '/dom/html',  # For nsHTMLDocument
diff --git a/gfx/layers/apz/util/DoubleTapToZoom.cpp b/gfx/layers/apz/util/DoubleTapToZoom.cpp
index 0ee5328367d1..705be137d2ab 100644
--- a/gfx/layers/apz/util/DoubleTapToZoom.cpp
+++ b/gfx/layers/apz/util/DoubleTapToZoom.cpp
@@ -31,8 +31,8 @@ using FrameForPointOption = nsLayoutUtils::FrameForPointOption;
 // the root frame of |aShell|. If the point is inside a subdocument, returns
 // an element inside the subdocument, rather than the subdocument element
 // (and does so recursively).
-// The implementation was adapted from nsDocument::ElementFromPoint(), with
-// the notable exception that we don't pass nsLayoutUtils::IGNORE_CROSS_DOC
+// The implementation was adapted from DocumentOrShadowRoot::ElementFromPoint(),
+// with the notable exception that we don't pass nsLayoutUtils::IGNORE_CROSS_DOC
 // to GetFrameForPoint(), so as to get the behaviour described above in the
 // presence of subdocuments.
 static already_AddRefed<dom::Element> ElementFromPoint(
diff --git a/image/SVGDocumentWrapper.cpp b/image/SVGDocumentWrapper.cpp
index 3f2bd1e228ce..bd0d49ee1396 100644
--- a/image/SVGDocumentWrapper.cpp
+++ b/image/SVGDocumentWrapper.cpp
@@ -28,7 +28,7 @@
 #include "mozilla/dom/SVGAnimatedLength.h"
 #include "nsMimeTypes.h"
 #include "DOMSVGLength.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "mozilla/dom/ImageTracker.h"
 
 namespace mozilla {
diff --git a/intl/l10n/DocumentL10n.h b/intl/l10n/DocumentL10n.h
index bae5e9e136c2..5301ba6c69c8 100644
--- a/intl/l10n/DocumentL10n.h
+++ b/intl/l10n/DocumentL10n.h
@@ -43,13 +43,13 @@ class PromiseResolver final : public PromiseNativeHandler {
 enum class DocumentL10nState { Initialized = 0, InitialTranslationTriggered };
 
 /**
- * This class maintains localization status of the nsDocument.
+ * This class maintains localization status of the document.
  *
- * The nsDocument will initialize it lazily when a link with a
- * localization resource is added to the document.
+ * The nsIDocument will initialize it lazily when a link with a localization
+ * resource is added to the document.
  *
  * Once initialized, DocumentL10n relays all API methods to an
- * instance of mozIDOMLocalization and maintaines a single promise
+ * instance of mozIDOMLocalization and maintains a single promise
  * which gets resolved the first time the document gets translated.
  */
 class DocumentL10n final : public nsIDOMEventListener, public nsWrapperCache {
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index 8fc50f28d61c..467a0a49e0fa 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -4035,7 +4035,7 @@ void PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
   bool didLayoutFlush = false;
   if (isSafeToFlush) {
     // Record that we are in a flush, so that our optimization in
-    // nsDocument::FlushPendingNotifications doesn't skip any re-entrant
+    // nsIDocument::FlushPendingNotifications doesn't skip any re-entrant
     // calls to us.  Otherwise, we might miss some needed flushes, since
     // we clear mNeedStyleFlush / mNeedLayoutFlush here at the top of
     // the function but we might not have done the work yet.
@@ -4047,7 +4047,7 @@ void PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
     // need the frames in the external document to be constructed for the
     // filter to work). We only need external resources to be flushed when the
     // main document is flushing >= FlushType::Frames, so we flush external
-    // resources here instead of nsDocument::FlushPendingNotifications.
+    // resources here instead of nsIDocument::FlushPendingNotifications.
     mDocument->FlushExternalResources(flushType);
 
     // Force flushing of any pending content notifications that might have
diff --git a/layout/base/ZoomConstraintsClient.cpp b/layout/base/ZoomConstraintsClient.cpp
index 1da811cd002c..2c6fbece1d8c 100644
--- a/layout/base/ZoomConstraintsClient.cpp
+++ b/layout/base/ZoomConstraintsClient.cpp
@@ -15,7 +15,7 @@
 #include "mozilla/Preferences.h"
 #include "mozilla/PresShell.h"
 #include "mozilla/dom/Event.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsIFrame.h"
 #include "nsLayoutUtils.h"
 #include "nsPoint.h"
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 5e21f430aa85..0b117dc162e4 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -2578,7 +2578,7 @@ NS_IMETHODIMP nsDocumentViewer::ClearSelection() {
 
 NS_IMETHODIMP nsDocumentViewer::SelectAll() {
   // XXX this is a temporary implementation copied from nsWebShell
-  // for now. I think nsDocument and friends should have some helper
+  // for now. I think nsIDocument and friends should have some helper
   // functions to make this easier.
 
   // use nsCopySupport::GetSelectionForCopy() ?
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 5150c3762852..3710139f69e3 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -24,7 +24,7 @@
 #include "mozilla/StaticPrefs.h"
 #include "mozilla/Unused.h"
 #include "nsCharTraits.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsFontMetrics.h"
 #include "nsPresContext.h"
 #include "nsIContent.h"
diff --git a/layout/inspector/ServoStyleRuleMap.cpp b/layout/inspector/ServoStyleRuleMap.cpp
index dabbbb1a4663..cec81184bbb8 100644
--- a/layout/inspector/ServoStyleRuleMap.cpp
+++ b/layout/inspector/ServoStyleRuleMap.cpp
@@ -14,7 +14,7 @@
 #include "mozilla/IntegerRange.h"
 #include "mozilla/ServoStyleSet.h"
 #include "mozilla/StyleSheetInlines.h"
-#include "nsDocument.h"
+#include "nsIDocument.h"
 #include "nsStyleSheetService.h"
 #include "nsXBLPrototypeResources.h"
 
diff --git a/toolkit/content/contentAreaUtils.js b/toolkit/content/contentAreaUtils.js
index ff541547c64b..8e83c5d7963b 100644
--- a/toolkit/content/contentAreaUtils.js
+++ b/toolkit/content/contentAreaUtils.js
@@ -193,7 +193,7 @@ function saveDocument(aDocument, aSkipPrompt) {
     contentDisposition = aDocument.contentDisposition;
     cacheKey = aDocument.cacheKey;
   } else if (aDocument.nodeType == 9 /* DOCUMENT_NODE */) {
-    // Otherwise it's an actual nsDocument (and possibly a CPOW).
+    // Otherwise it's an actual document (and possibly a CPOW).
     // We want to use cached data because the document is currently visible.
     let win = aDocument.defaultView;