diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 85c850c2257b..ebacad983245 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -211,6 +211,7 @@
 #include "mozilla/dom/ClientInfo.h"
 #include "mozilla/dom/ClientState.h"
 #include "mozilla/dom/DocumentFragment.h"
+#include "mozilla/dom/DocumentL10n.h"
 #include "mozilla/dom/DocumentTimeline.h"
 #include "mozilla/dom/Event.h"
 #include "mozilla/dom/HTMLBodyElement.h"
@@ -1885,6 +1886,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(nsDocument)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDisplayDocument)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mFontFaceSet)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mReadyForIdle)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDocumentL10n)
 
   // Traverse all nsDocument nsCOMPtrs.
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mParser)
@@ -2031,6 +2033,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsDocument)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mFontFaceSet)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mReadyForIdle);
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mCommandDispatcher)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mDocumentL10n);
 
   tmp->mParentDocument = nullptr;
 
@@ -2229,6 +2232,23 @@ nsIDocument::Reset(nsIChannel* aChannel, nsILoadGroup* aLoadGroup)
   mChannel = aChannel;
 }
 
+/**
+ * DocumentL10n is currently allowed for system
+ * principal.
+ *
+ * In the future we'll want to expose it to non-web-exposed
+ * about:* pages.
+ */
+bool
+PrincipalAllowsL10n(nsIPrincipal* principal)
+{
+  if (nsContentUtils::IsSystemPrincipal(principal)) {
+    return true;
+  }
+
+  return false;
+}
+
 void
 nsIDocument::ResetToURI(nsIURI* aURI,
                         nsILoadGroup* aLoadGroup,
@@ -3303,6 +3323,102 @@ nsIDocument::GetAllowPlugins()
   return true;
 }
 
+void
+nsIDocument::InitializeLocalization(nsTArray<nsString>& aResourceIds)
+{
+  MOZ_ASSERT(!mDocumentL10n, "mDocumentL10n should not be initialized yet");
+
+  DocumentL10n* l10n = new DocumentL10n(this);
+  MOZ_ALWAYS_TRUE(l10n->Init(aResourceIds));
+  mDocumentL10n = l10n;
+}
+
+DocumentL10n*
+nsIDocument::GetL10n()
+{
+  return mDocumentL10n;
+}
+
+bool
+nsDocument::DocumentSupportsL10n(JSContext* aCx, JSObject* aObject)
+{
+  return PrincipalAllowsL10n(nsContentUtils::SubjectPrincipal(aCx));
+}
+
+void
+nsIDocument::LocalizationLinkAdded(Element* aLinkElement)
+{
+  if (!PrincipalAllowsL10n(NodePrincipal())) {
+    return;
+  }
+
+  nsAutoString href;
+  aLinkElement->GetAttr(kNameSpaceID_None, nsGkAtoms::href, href);
+  // If the link is added after the DocumentL10n instance
+  // has been initialized, just pass the resource ID to it.
+  if (mDocumentL10n) {
+    AutoTArray<nsString, 1> resourceIds;
+    resourceIds.AppendElement(href);
+    mDocumentL10n->AddResourceIds(resourceIds);
+  } else {
+    // Otherwise, we're still parsing the document.
+    // In that case, add it to the pending list. This list
+    // will be resolved once the end of l10n resource
+    // container is reached.
+    mL10nResources.AppendElement(href);
+  }
+}
+
+void
+nsIDocument::LocalizationLinkRemoved(Element* aLinkElement)
+{
+  if (!PrincipalAllowsL10n(NodePrincipal())) {
+    return;
+  }
+
+  nsAutoString href;
+  aLinkElement->GetAttr(kNameSpaceID_None, nsGkAtoms::href, href);
+  if (mDocumentL10n) {
+    AutoTArray<nsString, 1> resourceIds;
+    resourceIds.AppendElement(href);
+    uint32_t remaining = mDocumentL10n->RemoveResourceIds(resourceIds);
+    if (remaining == 0) {
+      mDocumentL10n = nullptr;
+    }
+  } else {
+    mL10nResources.RemoveElement(href);
+  }
+}
+
+/**
+ * This method should be called once the end of the l10n
+ * resource container has been parsed.
+ *
+ * In XUL this is the end of the first </linkset>,
+ * In XHTML/HTML this is the end of </head>.
+ *
+ * This milestone is used to allow for batch
+ * localization context I/O and building done
+ * once when all resources in the document have been
+ * collected.
+ */
+void
+nsIDocument::OnL10nResourceContainerParsed()
+{
+  if (!mL10nResources.IsEmpty()) {
+    InitializeLocalization(mL10nResources);
+    mL10nResources.Clear();
+  }
+}
+
+void
+nsIDocument::TriggerInitialDocumentTranslation()
+{
+  if (mDocumentL10n) {
+    mDocumentL10n->TriggerInitialDocumentTranslation();
+  }
+}
+
 bool
 nsDocument::IsWebAnimationsEnabled(JSContext* aCx, JSObject* /*unused*/)
 {
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 1a0d85d181ef..ea3cb4cf6cee 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -158,6 +158,7 @@ public:
 
   virtual void StopDocumentLoad() override;
 
+  static bool DocumentSupportsL10n(JSContext* aCx, JSObject* aObject);
   static bool IsWebAnimationsEnabled(JSContext* aCx, JSObject* aObject);
   static bool IsWebAnimationsEnabled(mozilla::dom::CallerType aCallerType);
   static bool IsWebAnimationsGetAnimationsEnabled(JSContext* aCx,
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index b93a72945826..824491fd9083 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -147,6 +147,7 @@ class CDATASection;
 class Comment;
 struct CustomElementDefinition;
 class DocGroup;
+class DocumentL10n;
 class DocumentFragment;
 class DocumentTimeline;
 class DocumentType;
@@ -3562,6 +3563,69 @@ public:
   // For more information on Flash classification, see
   // toolkit/components/url-classifier/flash-block-lists.rst
   mozilla::dom::FlashClassification DocumentFlashClassification();
+
+  /**
+   * Localization
+   *
+   * For more information on DocumentL10n see
+   * intl/l10n/docs/fluent_tutorial.rst
+   */
+
+public:
+  /**
+   * This is a public method exposed on Document WebIDL
+   * to chrome only documents.
+   */
+  mozilla::dom::DocumentL10n* GetL10n();
+
+  /**
+   * This method should be called when the container
+   * of l10n resources parsing is completed.
+   *
+   * It triggers initial async fetch of the resources
+   * as early as possible.
+   *
+   * In HTML case this is </head>.
+   * In XUL case this is </linkset>.
+   */
+  void OnL10nResourceContainerParsed();
+
+  /**
+   * This method should be called when a link element
+   * with rel="localization" is being added to the
+   * l10n resource container element.
+   */
+  void LocalizationLinkAdded(Element* aLinkElement);
+
+  /**
+   * This method should be called when a link element
+   * with rel="localization" is being removed.
+   */
+  void LocalizationLinkRemoved(Element* aLinkElement);
+
+protected:
+  /**
+   * This method should be collect as soon as the
+   * parsing of the document is completed.
+   *
+   * In HTML this happens when readyState becomes
+   * `interactive`.
+   * In XUL it happens at `DoneWalking`, during
+   * `MozBeforeInitialXULLayout`.
+   *
+   * It triggers the initial translation of the
+   * document.
+   */
+  void TriggerInitialDocumentTranslation();
+
+  RefPtr<mozilla::dom::DocumentL10n> mDocumentL10n;
+
+private:
+  void InitializeLocalization(nsTArray<nsString>& aResourceIds);
+
+  nsTArray<nsString> mL10nResources;
+
+public:
   bool IsThirdParty();
 
   bool IsScopedStyleEnabled();
diff --git a/dom/webidl/Document.webidl b/dom/webidl/Document.webidl
index 7468786dd581..b7290dd84c44 100644
--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -525,6 +525,10 @@ partial interface Document {
   [ChromeOnly] readonly attribute unsigned long numTrackersBlocked;
 };
 
+partial interface Document {
+  [Func="nsDocument::DocumentSupportsL10n"] readonly attribute DocumentL10n? l10n;
+};
+
 Document implements XPathEvaluator;
 Document implements GlobalEventHandlers;
 Document implements DocumentAndElementEventHandlers;