diff --git a/dom/base/DOMIntersectionObserver.cpp b/dom/base/DOMIntersectionObserver.cpp
index 76411b4b6f78..911884878411 100644
--- a/dom/base/DOMIntersectionObserver.cpp
+++ b/dom/base/DOMIntersectionObserver.cpp
@@ -647,7 +647,7 @@ IntersectionInput DOMIntersectionObserver::ComputeInput(
 // https://w3c.github.io/IntersectionObserver/#update-intersection-observations-algo
 // (steps 2.1 - 2.5)
 IntersectionOutput DOMIntersectionObserver::Intersect(
-    const IntersectionInput& aInput, Element& aTarget,
+    const IntersectionInput& aInput, const Element& aTarget,
     IsContentVisibilityObserver aIsContentVisibilityObserver) {
   const bool isSimilarOrigin = SimilarOrigin(aTarget, aInput.mRootNode) ==
                                BrowsingContextOrigin::Similar;
diff --git a/dom/base/DOMIntersectionObserver.h b/dom/base/DOMIntersectionObserver.h
index 5b7273907eca..bdb371766386 100644
--- a/dom/base/DOMIntersectionObserver.h
+++ b/dom/base/DOMIntersectionObserver.h
@@ -154,7 +154,7 @@ class DOMIntersectionObserver final : public nsISupports,
 
   enum class IsContentVisibilityObserver : bool { No, Yes };
   static IntersectionOutput Intersect(
-      const IntersectionInput&, Element&,
+      const IntersectionInput&, const Element&,
       IsContentVisibilityObserver = IsContentVisibilityObserver::No);
   // Intersects with a given rect, already relative to the root frame.
   static IntersectionOutput Intersect(const IntersectionInput&, const nsRect&);
@@ -168,6 +168,9 @@ class DOMIntersectionObserver final : public nsISupports,
   static already_AddRefed<DOMIntersectionObserver>
   CreateContentVisibilityObserver(Document&);
 
+  static Maybe<nsRect> EdgeInclusiveIntersection(const nsRect& aRect,
+                                                 const nsRect& aOtherRect);
+
  protected:
   void Connect();
   void QueueIntersectionObserverEntry(Element* aTarget,
diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index 20073e552558..f02dc61def63 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -2539,6 +2539,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(Document)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mAll)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDocGroup)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mFrameRequestManager)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mContentIdentifiersForLCP)
 
   // Traverse all our nsCOMArrays.
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPreloadingImages)
@@ -2664,6 +2665,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Document)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mAll)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mReferrerInfo)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mPreloadReferrerInfo)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mContentIdentifiersForLCP);
 
   if (tmp->mDocGroup && tmp->mDocGroup->GetBrowsingContextGroup()) {
     tmp->mDocGroup->GetBrowsingContextGroup()->RemoveDocument(tmp,
diff --git a/dom/base/Document.h b/dom/base/Document.h
index 41feb0a06af5..8626699e8264 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -53,6 +53,7 @@
 #include "mozilla/dom/RadioGroupContainer.h"
 #include "mozilla/dom/TreeOrderedArray.h"
 #include "mozilla/dom/ViewportMetaData.h"
+#include "mozilla/dom/LargestContentfulPaint.h"
 #include "mozilla/glean/GleanMetrics.h"
 #include "nsAtom.h"
 #include "nsCOMArray.h"
@@ -2428,6 +2429,10 @@ class Document : public nsINode,
 
   LinkedList<MediaQueryList>& MediaQueryLists() { return mDOMMediaQueryLists; }
 
+  nsTHashtable<LCPEntryHashEntry>& ContentIdentifiersForLCP() {
+    return mContentIdentifiersForLCP;
+  }
+
   /**
    * Get the compatibility mode for this document
    */
@@ -5043,6 +5048,11 @@ class Document : public nsINode,
   // Our live MediaQueryLists
   LinkedList<MediaQueryList> mDOMMediaQueryLists;
 
+  // A hashset to keep track of which {element, imgRequestProxy}
+  // combination has been processed to avoid considering the same
+  // element twice for LargestContentfulPaint.
+  nsTHashtable<LCPEntryHashEntry> mContentIdentifiersForLCP;
+
   // Array of observers
   nsTObserverArray<nsIDocumentObserver*> mObservers;
 
diff --git a/dom/base/Element.h b/dom/base/Element.h
index b71c363f224f..a252040aeba6 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -181,8 +181,13 @@ enum : uint32_t {
   // element or has a HTML datalist element ancestor.
   ELEMENT_IS_DATALIST_OR_HAS_DATALIST_ANCESTOR = ELEMENT_FLAG_BIT(4),
 
+  // If this flag is set on an element, that means this element
+  // has been considered by our LargestContentfulPaint algorithm and
+  // it's not going to be considered again.
+  ELEMENT_PROCESSED_BY_LCP_FOR_TEXT = ELEMENT_FLAG_BIT(5),
+
   // Remaining bits are for subclasses
-  ELEMENT_TYPE_SPECIFIC_BITS_OFFSET = NODE_TYPE_SPECIFIC_BITS_OFFSET + 5
+  ELEMENT_TYPE_SPECIFIC_BITS_OFFSET = NODE_TYPE_SPECIFIC_BITS_OFFSET + 6
 };
 
 #undef ELEMENT_FLAG_BIT
diff --git a/dom/base/nsImageLoadingContent.cpp b/dom/base/nsImageLoadingContent.cpp
index 74a5b589f00f..c1320a3472cb 100644
--- a/dom/base/nsImageLoadingContent.cpp
+++ b/dom/base/nsImageLoadingContent.cpp
@@ -55,6 +55,7 @@
 #include "mozilla/dom/ScriptSettings.h"
 #include "mozilla/intl/LocaleService.h"
 #include "mozilla/intl/Locale.h"
+#include "mozilla/dom/LargestContentfulPaint.h"
 #include "mozilla/net/UrlClassifierFeatureFactory.h"
 #include "mozilla/widget/TextRecognition.h"
 
@@ -254,9 +255,11 @@ void nsImageLoadingContent::OnLoadComplete(imgIRequest* aRequest,
     FireEvent(u"error"_ns);
   }
 
-  SVGObserverUtils::InvalidateDirectRenderingObservers(
-      AsContent()->AsElement());
+  Element* element = AsContent()->AsElement();
+  SVGObserverUtils::InvalidateDirectRenderingObservers(element);
   MaybeResolveDecodePromises();
+  LargestContentfulPaint::MaybeProcessImageForElementTiming(mCurrentRequest,
+                                                            element);
 }
 
 void nsImageLoadingContent::OnUnlockedDraw() {
diff --git a/dom/events/EventDispatcher.cpp b/dom/events/EventDispatcher.cpp
index a70f2ccb0db5..03dc3e9dc140 100644
--- a/dom/events/EventDispatcher.cpp
+++ b/dom/events/EventDispatcher.cpp
@@ -40,6 +40,7 @@
 #include "mozilla/dom/NotifyPaintEvent.h"
 #include "mozilla/dom/PageTransitionEvent.h"
 #include "mozilla/dom/PerformanceEventTiming.h"
+#include "mozilla/dom/PerformanceMainThread.h"
 #include "mozilla/dom/PointerEvent.h"
 #include "mozilla/dom/RootedDictionary.h"
 #include "mozilla/dom/ScrollAreaEvent.h"
@@ -838,6 +839,14 @@ nsresult EventDispatcher::Dispatch(EventTarget* aTarget,
   if (aPresContext && !aPresContext->IsPrintingOrPrintPreview()) {
     eventTimingEntry =
         PerformanceEventTiming::TryGenerateEventTiming(target, aEvent);
+
+    if (aEvent->IsTrusted() && aEvent->mMessage == eScroll) {
+      if (auto* perf = aPresContext->GetPerformanceMainThread()) {
+        if (!perf->HasDispatchedScrollEvent()) {
+          perf->SetHasDispatchedScrollEvent();
+        }
+      }
+    }
   }
 
   bool retargeted = false;
diff --git a/dom/performance/LargestContentfulPaint.cpp b/dom/performance/LargestContentfulPaint.cpp
index 29984ade73d1..1607d1f12701 100644
--- a/dom/performance/LargestContentfulPaint.cpp
+++ b/dom/performance/LargestContentfulPaint.cpp
@@ -5,15 +5,30 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 #include "mozilla/dom/Element.h"
 #include "nsContentUtils.h"
+#include "nsLayoutUtils.h"
 #include "nsRFPService.h"
 #include "Performance.h"
+#include "imgRequest.h"
 #include "PerformanceMainThread.h"
 #include "LargestContentfulPaint.h"
 
+#include "mozilla/dom/DOMIntersectionObserver.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+
+#include "mozilla/PresShell.h"
+#include "mozilla/Logging.h"
+#include "mozilla/nsVideoFrame.h"
+
 namespace mozilla::dom {
 
+static LazyLogModule gLCPLogging("LargestContentfulPaint");
+
+#define LOG(...) MOZ_LOG(gLCPLogging, LogLevel::Debug, (__VA_ARGS__))
+
 NS_IMPL_CYCLE_COLLECTION_INHERITED(LargestContentfulPaint, PerformanceEntry,
-                                   mPerformance, mURI, mElement)
+                                   mPerformance, mURI, mElement,
+                                   mLCPImageEntryKey)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(LargestContentfulPaint)
 NS_INTERFACE_MAP_END_INHERITING(PerformanceEntry)
@@ -21,19 +36,34 @@ NS_INTERFACE_MAP_END_INHERITING(PerformanceEntry)
 NS_IMPL_ADDREF_INHERITED(LargestContentfulPaint, PerformanceEntry)
 NS_IMPL_RELEASE_INHERITED(LargestContentfulPaint, PerformanceEntry)
 
-LargestContentfulPaint::LargestContentfulPaint(Performance* aPerformance,
-                                               DOMHighResTimeStamp aRenderTime,
-                                               DOMHighResTimeStamp aLoadTime,
-                                               unsigned long aSize,
-                                               nsIURI* aURI, Element* aElement)
+static double GetAreaInDoublePixelsFromAppUnits(const nsSize& aSize) {
+  return NSAppUnitsToDoublePixels(aSize.Width(), AppUnitsPerCSSPixel()) *
+         NSAppUnitsToDoublePixels(aSize.Height(), AppUnitsPerCSSPixel());
+}
+
+static double GetAreaInDoublePixelsFromAppUnits(const nsRect& aRect) {
+  return NSAppUnitsToDoublePixels(aRect.Width(), AppUnitsPerCSSPixel()) *
+         NSAppUnitsToDoublePixels(aRect.Height(), AppUnitsPerCSSPixel());
+}
+
+ImagePendingRendering::ImagePendingRendering(
+    const LCPImageEntryKey& aLCPImageEntryKey, DOMHighResTimeStamp aLoadTime)
+    : mLCPImageEntryKey(aLCPImageEntryKey), mLoadTime(aLoadTime) {}
+
+LargestContentfulPaint::LargestContentfulPaint(
+    PerformanceMainThread* aPerformance, const DOMHighResTimeStamp aRenderTime,
+    const DOMHighResTimeStamp aLoadTime, const unsigned long aSize,
+    nsIURI* aURI, Element* aElement,
+    const Maybe<const LCPImageEntryKey>& aLCPImageEntryKey)
     : PerformanceEntry(aPerformance->GetParentObject(), u""_ns,
                        kLargestContentfulPaintName),
-      mPerformance(static_cast<PerformanceMainThread*>(aPerformance)),
+      mPerformance(aPerformance),
       mRenderTime(aRenderTime),
       mLoadTime(aLoadTime),
       mSize(aSize),
       mURI(aURI),
-      mElement(aElement) {
+      mElement(aElement),
+      mLCPImageEntryKey(aLCPImageEntryKey) {
   MOZ_ASSERT(mPerformance);
   MOZ_ASSERT(mElement);
   // The element could be a pseudo-element
@@ -58,6 +88,187 @@ Element* LargestContentfulPaint::GetElement() const {
                   : nullptr;
 }
 
+void LargestContentfulPaint::BufferEntryIfNeeded() {
+  mPerformance->BufferLargestContentfulPaintEntryIfNeeded(this);
+}
+
+/* static*/
+bool LCPHelpers::IsQualifiedImageRequest(imgRequest* aRequest,
+                                         Element* aContainingElement) {
+  MOZ_ASSERT(aContainingElement);
+  if (!aRequest) {
+    return false;
+  }
+
+  if (aRequest->IsChrome()) {
+    return false;
+  }
+
+  if (!aContainingElement->ChromeOnlyAccess()) {
+    return true;
+  }
+
+  // Exception: this is a poster image of video element
+  if (nsIContent* parent = aContainingElement->GetParent()) {
+    nsVideoFrame* videoFrame = do_QueryFrame(parent->GetPrimaryFrame());
+    if (videoFrame && videoFrame->GetPosterImage() == aContainingElement) {
+      return true;
+    }
+  }
+
+  // Exception: CSS generated images
+  if (aContainingElement->IsInNativeAnonymousSubtree()) {
+    if (nsINode* rootParentOrHost =
+            aContainingElement
+                ->GetClosestNativeAnonymousSubtreeRootParentOrHost()) {
+      if (!rootParentOrHost->ChromeOnlyAccess()) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+void LargestContentfulPaint::MaybeProcessImageForElementTiming(
+    imgRequestProxy* aRequest, Element* aElement) {
+  if (!StaticPrefs::dom_enable_largest_contentful_paint()) {
+    return;
+  }
+
+  MOZ_ASSERT(aRequest);
+  imgRequest* request = aRequest->GetOwner();
+  if (!LCPHelpers::IsQualifiedImageRequest(request, aElement)) {
+    return;
+  }
+
+  Document* document = aElement->GetComposedDoc();
+  if (!document) {
+    return;
+  }
+
+  nsPresContext* pc =
+      aElement->GetPresContext(Element::PresContextFor::eForComposedDoc);
+  if (!pc) {
+    return;
+  }
+
+  PerformanceMainThread* performance = pc->GetPerformanceMainThread();
+  if (!performance) {
+    return;
+  }
+
+  if (MOZ_UNLIKELY(MOZ_LOG_TEST(gLCPLogging, LogLevel::Debug))) {
+    nsCOMPtr<nsIURI> uri;
+    aRequest->GetURI(getter_AddRefs(uri));
+    LOG("MaybeProcessImageForElementTiming, Element=%p, URI=%s, "
+        "performance=%p ",
+        aElement, uri ? uri->GetSpecOrDefault().get() : "", performance);
+  }
+
+  const LCPImageEntryKey entryKey = LCPImageEntryKey(aElement, aRequest);
+  if (!document->ContentIdentifiersForLCP().EnsureInserted(entryKey)) {
+    LOG("  The content identifier existed for element=%p and request=%p, "
+        "return.",
+        aElement, aRequest);
+    return;
+  }
+
+#ifdef DEBUG
+  uint32_t status = imgIRequest::STATUS_NONE;
+  aRequest->GetImageStatus(&status);
+  MOZ_ASSERT(status & imgIRequest::STATUS_LOAD_COMPLETE);
+#endif
+
+  // Here we are exposing the load time of the image which could be
+  // a privacy concern. The spec talks about it at
+  // https://wicg.github.io/element-timing/#sec-security
+  // TLDR: The similar metric can be obtained by ResourceTiming
+  // API and onload handlers already, so this is not exposing anything
+  // new.
+  DOMHighResTimeStamp nowTime =
+      performance->TimeStampToDOMHighResForRendering(TimeStamp::Now());
+
+  if (!request->IsData() && !request->ShouldReportRenderTimeForLCP()) {
+    // https://wicg.github.io/element-timing/#report-image-element-timing
+    LOG("  Added a pending image rendering (TAO FAILED)");
+    // For TAO failed requests, the renderTime is exposed as 0 for
+    // security reasons.
+    LCPHelpers::CreateLCPEntryForImage(performance, aElement, aRequest, nowTime,
+                                       0 /* aRenderTime */, entryKey);
+    return;
+  }
+
+  // Otherwise, add the triple (element, imageRequest, now) to root’s images
+  // pending rendering.
+  //
+  // At this point, the loadTime of the image is known, but
+  // the renderTime is unknown, so it's added to ImagesPendingRendering
+  // as a placeholder, and the corresponding LCP entry will be created
+  // when the renderTime is known.
+  LOG("  Added a pending image rendering (TAO PASSED)");
+  performance->AddImagesPendingRendering(
+      ImagePendingRendering{entryKey, nowTime});
+}
+
+bool LCPHelpers::CanFinalizeLCPEntry(const nsIFrame* aFrame) {
+  if (!StaticPrefs::dom_enable_largest_contentful_paint()) {
+    return false;
+  }
+
+  if (!aFrame) {
+    return false;
+  }
+
+  nsPresContext* presContext = aFrame->PresContext();
+  return !presContext->HasStoppedGeneratingLCP() &&
+         presContext->GetPerformanceMainThread();
+}
+
+void LCPHelpers::FinalizeLCPEntryForImage(
+    Element* aContainingBlock, imgRequestProxy* aImgRequestProxy,
+    const nsRect& aTargetRectRelativeToSelf) {
+  LOG("FinalizeLCPEntryForImage element=%p", aContainingBlock);
+  if (!aImgRequestProxy) {
+    return;
+  }
+
+  if (!IsQualifiedImageRequest(aImgRequestProxy->GetOwner(),
+                               aContainingBlock)) {
+    return;
+  }
+
+  nsIFrame* frame = aContainingBlock->GetPrimaryFrame();
+
+  if (!CanFinalizeLCPEntry(frame)) {
+    return;
+  }
+
+  PerformanceMainThread* performance =
+      frame->PresContext()->GetPerformanceMainThread();
+  MOZ_ASSERT(performance);
+
+  RefPtr<LargestContentfulPaint> entry =
+      performance->GetImageLCPEntry(aContainingBlock, aImgRequestProxy);
+  if (!entry) {
+    LOG("  No Image Entry");
+    return;
+  }
+  entry->UpdateSize(aContainingBlock, aTargetRectRelativeToSelf, performance,
+                    true);
+  // If area is less than or equal to document’s largest contentful paint size,
+  // return.
+  if (!performance->UpdateLargestContentfulPaintSize(entry->Size())) {
+    LOG(
+
+        "  This paint(%lu) is not greater than the largest paint (%lf)that "
+        "we've "
+        "reported so far, return",
+        entry->Size(), performance->GetLargestContentfulPaintSize());
+    return;
+  }
+
+  entry->QueueEntry();
+}
+
 DOMHighResTimeStamp LargestContentfulPaint::RenderTime() const {
   return nsRFPService::ReduceTimePrecisionAsMSecs(
       mRenderTime, mPerformance->GetRandomTimelineSeed(),
@@ -77,9 +288,228 @@ DOMHighResTimeStamp LargestContentfulPaint::StartTime() const {
       mPerformance->GetRTPCallerType());
 }
 
+/* static */
+Element* LargestContentfulPaint::GetContainingBlockForTextFrame(
+    const nsTextFrame* aTextFrame) {
+  nsIFrame* containingFrame = aTextFrame->GetContainingBlock();
+  MOZ_ASSERT(containingFrame);
+  return Element::FromNodeOrNull(containingFrame->GetContent());
+}
+
+void LargestContentfulPaint::QueueEntry() {
+  LOG("QueueEntry entry=%p", this);
+  mPerformance->QueueLargestContentfulPaintEntry(this);
+}
+
 void LargestContentfulPaint::GetUrl(nsAString& aUrl) {
   if (mURI) {
     CopyUTF8toUTF16(mURI->GetSpecOrDefault(), aUrl);
   }
 }
+
+void LargestContentfulPaint::UpdateSize(
+    const Element* aContainingBlock, const nsRect& aTargetRectRelativeToSelf,
+    const PerformanceMainThread* aPerformance, bool aIsImage) {
+  nsIFrame* frame = aContainingBlock->GetPrimaryFrame();
+  MOZ_ASSERT(frame);
+
+  nsIFrame* rootFrame = frame->PresShell()->GetRootFrame();
+  if (!rootFrame) {
+    return;
+  }
+
+  if (frame->Style()->IsInOpacityZeroSubtree()) {
+    LOG("  Opacity:0 return");
+    return;
+  }
+
+  // The following size computation is based on a pending pull request
+  // https://github.com/w3c/largest-contentful-paint/pull/99
+
+  // Let visibleDimensions be concreteDimensions, adjusted for positioning
+  // by object-position or background-position and element’s content box.
+  const nsRect& visibleDimensions = aTargetRectRelativeToSelf;
+
+  // Let clientContentRect be the smallest DOMRectReadOnly containing
+  // visibleDimensions with element’s transforms applied.
+  nsRect clientContentRect = nsLayoutUtils::TransformFrameRectToAncestor(
+      frame, visibleDimensions, rootFrame);
+
+  // Let intersectionRect be the value returned by the intersection rect
+  // algorithm using element as the target and viewport as the root.
+  // (From https://wicg.github.io/element-timing/#sec-report-image-element)
+  IntersectionInput input = DOMIntersectionObserver::ComputeInput(
+      *frame->PresContext()->Document(), rootFrame->GetContent(), nullptr);
+  const IntersectionOutput output =
+      DOMIntersectionObserver::Intersect(input, *aContainingBlock);
+
+  Maybe<nsRect> intersectionRect = output.mIntersectionRect;
+
+  if (intersectionRect.isNothing()) {
+    LOG("  The intersectionRect is nothing for Element=%p. return.",
+        aContainingBlock);
+    return;
+  }
+
+  // Let intersectingClientContentRect be the intersection of clientContentRect
+  // with intersectionRect.
+  Maybe<nsRect> intersectionWithContentRect =
+      clientContentRect.EdgeInclusiveIntersection(intersectionRect.value());
+
+  if (intersectionWithContentRect.isNothing()) {
+    LOG("  The intersectionWithContentRect is nothing for Element=%p. return.",
+        aContainingBlock);
+    return;
+  }
+
+  nsRect renderedRect = intersectionWithContentRect.value();
+
+  double area = GetAreaInDoublePixelsFromAppUnits(renderedRect);
+
+  double viewport = GetAreaInDoublePixelsFromAppUnits(input.mRootRect);
+
+  LOG("  Viewport = %f, RenderRect = %f.", viewport, area);
+  // We don't want to report things that take the entire viewport.
+  if (area >= viewport) {
+    LOG("  The renderedRect is at least same as the area of the "
+        "viewport for Element=%p, return.",
+        aContainingBlock);
+    return;
+  }
+
+  Maybe<nsSize> intrinsicSize = frame->GetIntrinsicSize().ToSize();
+  const bool hasIntrinsicSize = intrinsicSize && !intrinsicSize->IsEmpty();
+
+  if (aIsImage && hasIntrinsicSize) {
+    //  Let (naturalWidth, naturalHeight) be imageRequest’s natural dimension.
+    //  Let naturalArea be naturalWidth * naturalHeight.
+    double naturalArea =
+        GetAreaInDoublePixelsFromAppUnits(intrinsicSize.value());
+
+    LOG("  naturalArea = %f", naturalArea);
+
+    // Let boundingClientArea be clientContentRect’s width * clientContentRect’s
+    // height.
+    double boundingClientArea =
+        NSAppUnitsToDoublePixels(clientContentRect.Width(),
+                                 AppUnitsPerCSSPixel()) *
+        NSAppUnitsToDoublePixels(clientContentRect.Height(),
+                                 AppUnitsPerCSSPixel());
+    LOG("  boundingClientArea = %f", boundingClientArea);
+
+    // Let scaleFactor be boundingClientArea / naturalArea.
+    double scaleFactor = boundingClientArea / naturalArea;
+    LOG("  scaleFactor = %f", scaleFactor);
+
+    // If scaleFactor is greater than 1, then divide area by scaleFactor.
+    if (scaleFactor > 1) {
+      LOG("  area before sacled doown %f", area);
+      area = area / scaleFactor;
+    }
+  }
+
+  MOZ_ASSERT(!mSize);
+  mSize = area;
+}
+
+void LCPTextFrameHelper::MaybeUnionTextFrame(
+    nsTextFrame* aTextFrame, const nsRect& aRelativeToSelfRect) {
+  if (!StaticPrefs::dom_enable_largest_contentful_paint() ||
+      aTextFrame->PresContext()->HasStoppedGeneratingLCP()) {
+    return;
+  }
+
+  Element* containingBlock =
+      LargestContentfulPaint::GetContainingBlockForTextFrame(aTextFrame);
+  if (!containingBlock ||
+      // If element is contained in doc’s set of elements with rendered text,
+      // continue
+      containingBlock->HasFlag(ELEMENT_PROCESSED_BY_LCP_FOR_TEXT) ||
+      containingBlock->ChromeOnlyAccess()) {
+    return;
+  }
+
+  MOZ_ASSERT(containingBlock->GetPrimaryFrame());
+
+  PerformanceMainThread* perf =
+      aTextFrame->PresContext()->GetPerformanceMainThread();
+  if (!perf) {
+    return;
+  }
+
+  auto& unionRect = perf->GetTextFrameUnions().LookupOrInsert(containingBlock);
+  unionRect = unionRect.Union(aRelativeToSelfRect);
+}
+
+void LCPHelpers::CreateLCPEntryForImage(
+    PerformanceMainThread* aPerformance, Element* aElement,
+    imgRequestProxy* aRequestProxy, const DOMHighResTimeStamp aLoadTime,
+    const DOMHighResTimeStamp aRenderTime,
+    const LCPImageEntryKey& aImageEntryKey) {
+  MOZ_ASSERT(StaticPrefs::dom_enable_largest_contentful_paint());
+  MOZ_ASSERT(aRequestProxy);
+  if (MOZ_UNLIKELY(MOZ_LOG_TEST(gLCPLogging, LogLevel::Debug))) {
+    nsCOMPtr<nsIURI> uri;
+    aRequestProxy->GetURI(getter_AddRefs(uri));
+    LOG("CreateLCPEntryForImage "
+        "Element=%p, aRequestProxy=%p, URI=%s loadTime=%f, "
+        "aRenderTime=%f\n",
+        aElement, aRequestProxy, uri->GetSpecOrDefault().get(), aLoadTime,
+        aRenderTime);
+  }
+  MOZ_ASSERT(aPerformance);
+  MOZ_ASSERT_IF(aRenderTime < aLoadTime, aRenderTime == 0);
+  if (aPerformance->HasDispatchedInputEvent() ||
+      aPerformance->HasDispatchedScrollEvent()) {
+    return;
+  }
+
+  // Let url be the empty string.
+  // If imageRequest is not null, set url to be imageRequest’s request URL.
+  nsCOMPtr<nsIURI> requestURI;
+  aRequestProxy->GetURI(getter_AddRefs(requestURI));
+
+  // At this point, we have all the information about the entry
+  // except the size.
+  RefPtr<LargestContentfulPaint> entry =
+      new LargestContentfulPaint(aPerformance, aRenderTime, aLoadTime, 0,
+                                 requestURI, aElement, Some(aImageEntryKey));
+
+  LOG("  Upsert a LargestContentfulPaint entry=%p to LCPEntryMap.",
+      entry.get());
+  aPerformance->StoreImageLCPEntry(aElement, aRequestProxy, entry);
+}
+
+void LCPHelpers::FinalizeLCPEntryForText(
+    PerformanceMainThread* aPerformance, const DOMHighResTimeStamp aRenderTime,
+    Element* aContainingBlock, const nsRect& aTargetRectRelativeToSelf,
+    const nsPresContext* aPresContext) {
+  MOZ_ASSERT(aPerformance);
+  LOG("FinalizeLCPEntryForText element=%p", aContainingBlock);
+
+  if (!aContainingBlock->GetPrimaryFrame()) {
+    return;
+  }
+  MOZ_ASSERT(CanFinalizeLCPEntry(aContainingBlock->GetPrimaryFrame()));
+  MOZ_ASSERT(!aContainingBlock->HasFlag(ELEMENT_PROCESSED_BY_LCP_FOR_TEXT));
+  MOZ_ASSERT(!aContainingBlock->ChromeOnlyAccess());
+
+  aContainingBlock->SetFlags(ELEMENT_PROCESSED_BY_LCP_FOR_TEXT);
+
+  RefPtr<LargestContentfulPaint> entry = new LargestContentfulPaint(
+      aPerformance, aRenderTime, 0, 0, nullptr, aContainingBlock, Nothing());
+
+  entry->UpdateSize(aContainingBlock, aTargetRectRelativeToSelf, aPerformance,
+                    false);
+  // If area is less than or equal to document’s largest contentful paint size,
+  // return.
+  if (!aPerformance->UpdateLargestContentfulPaintSize(entry->Size())) {
+    LOG("  This paint(%lu) is not greater than the largest paint (%lf)that "
+        "we've "
+        "reported so far, return",
+        entry->Size(), aPerformance->GetLargestContentfulPaintSize());
+    return;
+  }
+  entry->QueueEntry();
+}
 }  // namespace mozilla::dom
diff --git a/dom/performance/LargestContentfulPaint.h b/dom/performance/LargestContentfulPaint.h
index e32eb37970a6..386b4d5ffee7 100644
--- a/dom/performance/LargestContentfulPaint.h
+++ b/dom/performance/LargestContentfulPaint.h
@@ -10,7 +10,8 @@
 #include "nsCycleCollectionParticipant.h"
 #include "mozilla/dom/PerformanceEntry.h"
 #include "mozilla/dom/PerformanceLargestContentfulPaintBinding.h"
-#include "nsIWeakReferenceUtils.h"
+
+#include "imgRequestProxy.h"
 
 class nsTextFrame;
 namespace mozilla::dom {
@@ -21,6 +22,143 @@ static constexpr nsLiteralString kLargestContentfulPaintName =
 class Performance;
 class PerformanceMainThread;
 
+struct LCPImageEntryKey {
+  LCPImageEntryKey(Element* aElement, imgRequestProxy* aImgRequestProxy)
+      : mElement(aElement), mImageRequestProxy(aImgRequestProxy) {
+    MOZ_ASSERT(aElement);
+    MOZ_ASSERT(aImgRequestProxy);
+  }
+
+  LCPImageEntryKey(const LCPImageEntryKey& aLCPImageEntryKey) {
+    mElement = aLCPImageEntryKey.mElement;
+    mImageRequestProxy = aLCPImageEntryKey.mImageRequestProxy;
+  }
+
+  bool operator==(const LCPImageEntryKey& aOther) const {
+    return mElement == aOther.mElement &&
+           mImageRequestProxy == aOther.mImageRequestProxy;
+  }
+
+  bool Equals(const Element* aElement,
+              const imgRequestProxy* aImgRequestProxy) const {
+    return mElement == mElement && mImageRequestProxy == mImageRequestProxy;
+  }
+
+  RefPtr<Element> mElement;
+  RefPtr<imgRequestProxy> mImageRequestProxy;
+
+  ~LCPImageEntryKey() = default;
+};
+
+inline void ImplCycleCollectionUnlink(LCPImageEntryKey& aField) {
+  aField.mElement = nullptr;
+  aField.mImageRequestProxy = nullptr;
+}
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, LCPImageEntryKey& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  ImplCycleCollectionTraverse(aCallback, aField.mElement,
+                              "LCPImageEntryKey.mElement", aFlags);
+  ImplCycleCollectionTraverse(aCallback, aField.mImageRequestProxy,
+                              "LCPImageEntryKey.mImageRequestProxy", aFlags);
+}
+
+struct LCPTextFrameHelper final {
+  static void MaybeUnionTextFrame(nsTextFrame* aTextFrame,
+                                  const nsRect& aRelativeToSelfRect);
+  static void HasContainingElementBeenProcessed();
+};
+
+class ImagePendingRendering final {
+ public:
+  ImagePendingRendering(const LCPImageEntryKey& aLCPImageEntryKey,
+                        DOMHighResTimeStamp aLoadTime);
+
+  Element* GetElement() const { return mLCPImageEntryKey.mElement; }
+
+  imgRequestProxy* GetImgRequestProxy() const {
+    return mLCPImageEntryKey.mImageRequestProxy;
+  }
+
+  LCPImageEntryKey mLCPImageEntryKey;
+  DOMHighResTimeStamp mLoadTime;
+};
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    ImagePendingRendering& aField, const char* aName, uint32_t aFlags = 0) {
+  ImplCycleCollectionTraverse(aCallback, aField.mLCPImageEntryKey,
+                              "ImagePendingRendering.mImageEntryKey",
+                              aCallback.Flags());
+}
+
+class LCPEntryHashEntry : public PLDHashEntryHdr {
+ public:
+  typedef const LCPImageEntryKey& KeyType;
+  typedef const LCPImageEntryKey* KeyTypePointer;
+
+  explicit LCPEntryHashEntry(KeyTypePointer aKey) : mKey(*aKey) {}
+  LCPEntryHashEntry(LCPEntryHashEntry&&) = default;
+
+  ~LCPEntryHashEntry() = default;
+
+  bool KeyEquals(KeyTypePointer aKey) const { return mKey == *aKey; }
+
+  KeyType GetKey() const { return mKey; }
+
+  static KeyTypePointer KeyToPointer(KeyType& aKey) { return &aKey; }
+
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    if (!aKey) {
+      return 0;
+    }
+
+    return mozilla::HashGeneric(
+        reinterpret_cast<uintptr_t>(aKey->mElement.get()),
+        reinterpret_cast<uintptr_t>(aKey->mImageRequestProxy.get()));
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+  LCPImageEntryKey mKey;
+};
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, LCPEntryHashEntry& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  ImplCycleCollectionTraverse(aCallback, aField.mKey, "LCPEntryHashEntry.mKey",
+                              aFlags);
+}
+
+class LCPHelpers final {
+ public:
+  // Creates the LCP Entry for images with all information except the size of
+  // the element. The size of the image is unknown at the moment. The entry is
+  // not going to be queued in this function.
+  static void CreateLCPEntryForImage(
+      PerformanceMainThread* aPerformance, Element* aElement,
+      imgRequestProxy* aRequestProxy, const DOMHighResTimeStamp aLoadTime,
+      const DOMHighResTimeStamp aRenderTime,
+      const LCPImageEntryKey& aContentIdentifier);
+
+  // Called when the size of the image is known.
+  static void FinalizeLCPEntryForImage(Element* aContainingBlock,
+                                       imgRequestProxy* aImgRequestProxy,
+                                       const nsRect& aTargetRectRelativeToSelf);
+
+  static void FinalizeLCPEntryForText(PerformanceMainThread* aPerformance,
+                                      const DOMHighResTimeStamp aRenderTime,
+                                      Element* aContainingBlock,
+                                      const nsRect& aTargetRectRelativeToSelf,
+                                      const nsPresContext* aPresContext);
+
+  static bool IsQualifiedImageRequest(imgRequest* aRequest,
+                                      Element* aContainingElement);
+
+ private:
+  static bool CanFinalizeLCPEntry(const nsIFrame* aFrame);
+};
+
 // https://w3c.github.io/largest-contentful-paint/
 class LargestContentfulPaint final : public PerformanceEntry {
  public:
@@ -29,10 +167,12 @@ class LargestContentfulPaint final : public PerformanceEntry {
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(LargestContentfulPaint,
                                            PerformanceEntry)
 
-  LargestContentfulPaint(Performance* aPerformance,
-                         DOMHighResTimeStamp aRenderTime,
-                         DOMHighResTimeStamp aLoadTime, unsigned long aSize,
-                         nsIURI* aURI, Element* aElement);
+  LargestContentfulPaint(
+      PerformanceMainThread* aPerformance,
+      const DOMHighResTimeStamp aRenderTime,
+      const DOMHighResTimeStamp aLoadTime, const unsigned long aSize,
+      nsIURI* aURI, Element* aElement,
+      const Maybe<const LCPImageEntryKey>& aLCPImageEntryKey);
 
   JSObject* WrapObject(JSContext* aCx,
                        JS::Handle<JSObject*> aGivenProto) override;
@@ -51,6 +191,23 @@ class LargestContentfulPaint final : public PerformanceEntry {
 
   Element* GetElement() const;
 
+  static Element* GetContainingBlockForTextFrame(const nsTextFrame* aTextFrame);
+
+  void UpdateSize(const Element* aContainingBlock,
+                  const nsRect& aTargetRectRelativeToSelf,
+                  const PerformanceMainThread* aPerformance, bool aIsImage);
+
+  void BufferEntryIfNeeded() override;
+
+  static void MaybeProcessImageForElementTiming(imgRequestProxy* aRequest,
+                                                Element* aElement);
+
+  void QueueEntry();
+
+  const Maybe<LCPImageEntryKey>& GetLCPImageEntryKey() const {
+    return mLCPImageEntryKey;
+  }
+
  private:
   ~LargestContentfulPaint() = default;
 
@@ -63,6 +220,8 @@ class LargestContentfulPaint final : public PerformanceEntry {
 
   RefPtr<Element> mElement;
   RefPtr<nsAtom> mId;
+
+  Maybe<LCPImageEntryKey> mLCPImageEntryKey;
 };
 }  // namespace mozilla::dom
 #endif
diff --git a/dom/performance/PerformanceMainThread.cpp b/dom/performance/PerformanceMainThread.cpp
index 1665849db2bc..d5bb0c85fd53 100644
--- a/dom/performance/PerformanceMainThread.cpp
+++ b/dom/performance/PerformanceMainThread.cpp
@@ -12,6 +12,7 @@
 #include "js/PropertyAndElement.h"  // JS_DefineProperty
 #include "mozilla/HoldDropJSObjects.h"
 #include "PerformanceEventTiming.h"
+#include "LargestContentfulPaint.h"
 #include "mozilla/dom/Document.h"
 #include "mozilla/dom/Event.h"
 #include "mozilla/dom/EventCounts.h"
@@ -25,9 +26,13 @@
 #include "nsIChannel.h"
 #include "nsIHttpChannel.h"
 #include "nsIDocShell.h"
+#include "nsTextFrame.h"
+#include "nsContainerFrame.h"
 
 namespace mozilla::dom {
 
+extern mozilla::LazyLogModule gLCPLogging;
+
 namespace {
 
 void GetURLSpecFromChannel(nsITimedChannel* aChannel, nsAString& aSpec) {
@@ -59,19 +64,24 @@ NS_IMPL_CYCLE_COLLECTION_CLASS(PerformanceMainThread)
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(PerformanceMainThread,
                                                 Performance)
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mTiming, mNavigation, mDocEntry, mFCPTiming,
-                                  mEventTimingEntries, mFirstInputEvent,
-                                  mPendingPointerDown,
-                                  mPendingEventTimingEntries, mEventCounts)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(
+      mTiming, mNavigation, mDocEntry, mFCPTiming, mEventTimingEntries,
+      mLargestContentfulPaintEntries, mFirstInputEvent, mPendingPointerDown,
+      mPendingEventTimingEntries, mEventCounts)
+  tmp->mImageLCPEntryMap.Clear();
+  tmp->mTextFrameUnions.Clear();
+  tmp->mImagesPendingRendering.Clear();
   mozilla::DropJSObjects(tmp);
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(PerformanceMainThread,
                                                   Performance)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mTiming, mNavigation, mDocEntry, mFCPTiming,
-                                    mEventTimingEntries, mFirstInputEvent,
-                                    mPendingPointerDown,
-                                    mPendingEventTimingEntries, mEventCounts)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(
+      mTiming, mNavigation, mDocEntry, mFCPTiming, mEventTimingEntries,
+      mLargestContentfulPaintEntries, mFirstInputEvent, mPendingPointerDown,
+      mPendingEventTimingEntries, mEventCounts, mImagesPendingRendering,
+      mImageLCPEntryMap, mTextFrameUnions)
+
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN_INHERITED(PerformanceMainThread,
@@ -241,6 +251,15 @@ void PerformanceMainThread::BufferEventTimingEntryIfNeeded(
   }
 }
 
+void PerformanceMainThread::BufferLargestContentfulPaintEntryIfNeeded(
+    LargestContentfulPaint* aEntry) {
+  MOZ_ASSERT(StaticPrefs::dom_enable_largest_contentful_paint());
+  if (mLargestContentfulPaintEntries.Length() <
+      kMaxLargestContentfulPaintBufferSize) {
+    mLargestContentfulPaintEntries.AppendElement(aEntry);
+  }
+}
+
 void PerformanceMainThread::DispatchPendingEventTimingEntries() {
   DOMHighResTimeStamp renderingTime = NowUnclamped();
 
@@ -267,7 +286,7 @@ void PerformanceMainThread::DispatchPendingEventTimingEntries() {
             MOZ_ASSERT(!mFirstInputEvent);
             mFirstInputEvent = mPendingPointerDown.forget();
             QueueEntry(mFirstInputEvent);
-            mHasDispatchedInputEvent = true;
+            SetHasDispatchedInputEvent();
           }
           break;
         }
@@ -277,7 +296,7 @@ void PerformanceMainThread::DispatchPendingEventTimingEntries() {
           mFirstInputEvent = entry->Clone();
           mFirstInputEvent->SetEntryType(u"first-input"_ns);
           QueueEntry(mFirstInputEvent);
-          mHasDispatchedInputEvent = true;
+          SetHasDispatchedInputEvent();
           break;
         }
         default:
@@ -449,6 +468,12 @@ void PerformanceMainThread::QueueNavigationTimingEntry() {
   QueueEntry(mDocEntry);
 }
 
+void PerformanceMainThread::QueueLargestContentfulPaintEntry(
+    LargestContentfulPaint* aEntry) {
+  MOZ_ASSERT(StaticPrefs::dom_enable_largest_contentful_paint());
+  QueueEntry(aEntry);
+}
+
 EventCounts* PerformanceMainThread::EventCounts() {
   MOZ_ASSERT(StaticPrefs::dom_enable_event_timing());
   return mEventCounts;
@@ -501,6 +526,14 @@ void PerformanceMainThread::GetEntriesByTypeForObserver(
     aRetval.AppendElements(mEventTimingEntries);
     return;
   }
+
+  if (StaticPrefs::dom_enable_largest_contentful_paint()) {
+    if (aEntryType.EqualsLiteral("largest-contentful-paint")) {
+      aRetval.AppendElements(mLargestContentfulPaintEntries);
+      return;
+    }
+  }
+
   return GetEntriesByType(aEntryType, aRetval);
 }
 
@@ -562,4 +595,129 @@ size_t PerformanceMainThread::SizeOfEventEntries(
   }
   return eventEntries;
 }
+
+void PerformanceMainThread::ProcessElementTiming() {
+  if (!StaticPrefs::dom_enable_largest_contentful_paint()) {
+    return;
+  }
+  const bool shouldLCPDataEmpty =
+      HasDispatchedInputEvent() || HasDispatchedScrollEvent();
+  MOZ_ASSERT_IF(shouldLCPDataEmpty,
+                mTextFrameUnions.IsEmpty() && mImageLCPEntryMap.IsEmpty());
+
+  if (shouldLCPDataEmpty) {
+    return;
+  }
+
+  nsPresContext* presContext = GetPresShell()->GetPresContext();
+  MOZ_ASSERT(presContext);
+
+  // After https://github.com/w3c/largest-contentful-paint/issues/104 is
+  // resolved, LargestContentfulPaint and FirstContentfulPaint should
+  // be using the same timestamp, which should be the same timestamp
+  // as to what https://w3c.github.io/paint-timing/#mark-paint-timing step 2
+  // defines.
+  // TODO(sefeng): Check the timestamp after this issue is resolved.
+  DOMHighResTimeStamp rawNowTime =
+      TimeStampToDOMHighResForRendering(presContext->GetMarkPaintTimingStart());
+
+  MOZ_ASSERT(GetOwnerGlobal());
+  Document* document = GetOwnerGlobal()->GetAsInnerWindow()->GetExtantDoc();
+  if (!document ||
+      !nsContentUtils::GetInProcessSubtreeRootDocument(document)->IsActive()) {
+    return;
+  }
+
+  nsTArray<ImagePendingRendering> imagesPendingRendering =
+      std::move(mImagesPendingRendering);
+  for (const auto& imagePendingRendering : imagesPendingRendering) {
+    RefPtr<Element> element = imagePendingRendering.GetElement();
+    if (!element) {
+      continue;
+    }
+
+    MOZ_ASSERT(imagePendingRendering.mLoadTime < rawNowTime);
+    if (imgRequestProxy* requestProxy =
+            imagePendingRendering.GetImgRequestProxy()) {
+      LCPHelpers::CreateLCPEntryForImage(
+          this, element, requestProxy, imagePendingRendering.mLoadTime,
+          rawNowTime, imagePendingRendering.mLCPImageEntryKey);
+    }
+  }
+
+  MOZ_ASSERT(mImagesPendingRendering.IsEmpty());
+}
+
+void PerformanceMainThread::FinalizeLCPEntriesForText() {
+  nsPresContext* presContext = GetPresShell()->GetPresContext();
+  MOZ_ASSERT(presContext);
+
+  DOMHighResTimeStamp renderTime =
+      TimeStampToDOMHighResForRendering(presContext->GetMarkPaintTimingStart());
+
+  bool canFinalize = StaticPrefs::dom_enable_largest_contentful_paint() &&
+                     !presContext->HasStoppedGeneratingLCP();
+  if (canFinalize) {
+    for (const auto& textFrameUnion : GetTextFrameUnions()) {
+      LCPHelpers::FinalizeLCPEntryForText(
+          this, renderTime, textFrameUnion.GetKey(), textFrameUnion.GetData(),
+          presContext);
+    }
+  }
+
+  ClearTextFrameUnions();
+}
+
+void PerformanceMainThread::StoreImageLCPEntry(
+    Element* aElement, imgRequestProxy* aImgRequestProxy,
+    LargestContentfulPaint* aEntry) {
+  mImageLCPEntryMap.InsertOrUpdate({aElement, aImgRequestProxy}, aEntry);
+}
+
+already_AddRefed<LargestContentfulPaint>
+PerformanceMainThread::GetImageLCPEntry(Element* aElement,
+                                        imgRequestProxy* aImgRequestProxy) {
+  Maybe<RefPtr<LargestContentfulPaint>> entry =
+      mImageLCPEntryMap.Extract({aElement, aImgRequestProxy});
+  if (entry.isNothing()) {
+    return nullptr;
+  }
+
+  Document* doc = aElement->GetComposedDoc();
+  MOZ_ASSERT(doc, "Element should be connected when it's painted");
+
+  const Maybe<const LCPImageEntryKey>& contentIdentifier =
+      entry.value()->GetLCPImageEntryKey();
+  if (contentIdentifier.isSome()) {
+    doc->ContentIdentifiersForLCP().EnsureRemoved(contentIdentifier.value());
+  }
+
+  return entry.value().forget();
+}
+
+bool PerformanceMainThread::UpdateLargestContentfulPaintSize(double aSize) {
+  if (aSize > mLargestContentfulPaintSize) {
+    mLargestContentfulPaintSize = aSize;
+    return true;
+  }
+  return false;
+}
+
+void PerformanceMainThread::SetHasDispatchedScrollEvent() {
+  mHasDispatchedScrollEvent = true;
+  ClearGeneratedTempDataForLCP();
+}
+
+void PerformanceMainThread::SetHasDispatchedInputEvent() {
+  mHasDispatchedInputEvent = true;
+  mImageLCPEntryMap.Clear();
+  ClearGeneratedTempDataForLCP();
+}
+
+void PerformanceMainThread::ClearTextFrameUnions() { mTextFrameUnions.Clear(); }
+
+void PerformanceMainThread::ClearGeneratedTempDataForLCP() {
+  ClearTextFrameUnions();
+  mImageLCPEntryMap.Clear();
+}
 }  // namespace mozilla::dom
diff --git a/dom/performance/PerformanceMainThread.h b/dom/performance/PerformanceMainThread.h
index 8a38a5faecc3..36d421da3023 100644
--- a/dom/performance/PerformanceMainThread.h
+++ b/dom/performance/PerformanceMainThread.h
@@ -9,12 +9,19 @@
 
 #include "Performance.h"
 #include "PerformanceStorage.h"
+#include "LargestContentfulPaint.h"
+#include "nsTextFrame.h"
 
 namespace mozilla::dom {
 
 class PerformanceNavigationTiming;
 class PerformanceEventTiming;
 
+using ImageLCPEntryMap =
+    nsTHashMap<LCPEntryHashEntry, RefPtr<LargestContentfulPaint>>;
+
+using TextFrameUnions = nsTHashMap<nsRefPtrHashKey<Element>, nsRect>;
+
 class PerformanceMainThread final : public Performance,
                                     public PerformanceStorage {
  public:
@@ -45,11 +52,14 @@ class PerformanceMainThread final : public Performance,
                    const nsAString& aInitiatorType,
                    const nsAString& aEntryName);
   virtual void SetFCPTimingEntry(PerformancePaintTiming* aEntry) override;
+  bool HadFCPTimingEntry() const { return mFCPTiming; }
 
   void InsertEventTimingEntry(PerformanceEventTiming*) override;
   void BufferEventTimingEntryIfNeeded(PerformanceEventTiming*) override;
   void DispatchPendingEventTimingEntries() override;
 
+  void BufferLargestContentfulPaintEntryIfNeeded(LargestContentfulPaint*);
+
   TimeStamp CreationTimeStamp() const override;
 
   DOMHighResTimeStamp CreationTime() const override;
@@ -87,6 +97,7 @@ class PerformanceMainThread final : public Performance,
 
   void UpdateNavigationTimingEntry() override;
   void QueueNavigationTimingEntry() override;
+  void QueueLargestContentfulPaintEntry(LargestContentfulPaint* aEntry);
 
   size_t SizeOfEventEntries(mozilla::MallocSizeOf aMallocSizeOf) const override;
 
@@ -94,10 +105,40 @@ class PerformanceMainThread final : public Performance,
   static constexpr uint32_t kDefaultEventTimingDurationThreshold = 104;
   static constexpr double kDefaultEventTimingMinDuration = 16.0;
 
+  static constexpr uint32_t kMaxLargestContentfulPaintBufferSize = 150;
+
   class EventCounts* EventCounts() override;
 
   bool IsGlobalObjectWindow() const override { return true; };
 
+  bool HasDispatchedInputEvent() const { return mHasDispatchedInputEvent; }
+
+  void SetHasDispatchedScrollEvent();
+  bool HasDispatchedScrollEvent() const { return mHasDispatchedScrollEvent; }
+
+  void ProcessElementTiming();
+
+  void AddImagesPendingRendering(ImagePendingRendering aImagePendingRendering) {
+    mImagesPendingRendering.AppendElement(aImagePendingRendering);
+  }
+
+  void StoreImageLCPEntry(Element* aElement, imgRequestProxy* aImgRequestProxy,
+                          LargestContentfulPaint* aEntry);
+
+  already_AddRefed<LargestContentfulPaint> GetImageLCPEntry(
+      Element* aElement, imgRequestProxy* aImgRequestProxy);
+
+  bool UpdateLargestContentfulPaintSize(double aSize);
+  double GetLargestContentfulPaintSize() const {
+    return mLargestContentfulPaintSize;
+  }
+
+  nsTHashMap<nsRefPtrHashKey<Element>, nsRect>& GetTextFrameUnions() {
+    return mTextFrameUnions;
+  }
+
+  void FinalizeLCPEntriesForText();
+
  protected:
   ~PerformanceMainThread();
 
@@ -119,23 +160,76 @@ class PerformanceMainThread final : public Performance,
   JS::Heap<JSObject*> mMozMemory;
 
   nsTArray<RefPtr<PerformanceEventTiming>> mEventTimingEntries;
+  nsTArray<RefPtr<LargestContentfulPaint>> mLargestContentfulPaintEntries;
 
   AutoCleanLinkedList<RefPtr<PerformanceEventTiming>>
       mPendingEventTimingEntries;
   bool mHasDispatchedInputEvent = false;
+  bool mHasDispatchedScrollEvent = false;
 
   RefPtr<PerformanceEventTiming> mFirstInputEvent;
   RefPtr<PerformanceEventTiming> mPendingPointerDown;
 
  private:
+  void ClearTextFrameUnions();
+  void ClearGeneratedTempDataForLCP();
+
+  void SetHasDispatchedInputEvent();
+
   bool mHasQueuedRefreshdriverObserver = false;
 
   RefPtr<class EventCounts> mEventCounts;
   void IncEventCount(const nsAtom* aType);
 
   PresShell* GetPresShell();
+
+  nsTArray<ImagePendingRendering> mImagesPendingRendering;
+
+  // The key is the pair of the element initiates the image loading
+  // and the imgRequestProxy of the image, and the value is
+  // the LCP entry for this image. When the image is
+  // completely loaded, we add it to mImageLCPEntryMap.
+  // Later, when the image is painted, we get the LCP entry from it
+  // to update the size and queue the entry if needed.
+  //
+  // When the initiating element is disconnected from the document,
+  // we keep the orphan entry because if the same memory address is
+  // reused by a different LCP candidate, it'll update
+  // mImageLCPEntryMap precedes before it tries to get the LCP entry.
+  ImageLCPEntryMap mImageLCPEntryMap;
+
+  // Keeps track of the rendered size of the largest contentful paint that
+  // we have processed so far.
+  double mLargestContentfulPaintSize = 0.0;
+
+  // When a text frame is painted, its area (relative to the
+  // containing block) is unioned with other text frames that
+  // belong to the same containing block.
+  // mTextFrameUnions's key is the containing block, and
+  // the value is the unioned area.
+  TextFrameUnions mTextFrameUnions;
 };
 
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, ImageLCPEntryMap& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  for (auto& entry : aField) {
+    ImplCycleCollectionTraverse(aCallback, entry.mKey, "ImageLCPEntryMap.mKey", aCallback.Flags());
+    RefPtr<LargestContentfulPaint>* lcpEntry = entry.GetModifiableData();
+    ImplCycleCollectionTraverse(aCallback, *lcpEntry, "ImageLCPEntryMap.mData", aCallback.Flags());
+  }
+}
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, TextFrameUnions& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  for (auto& entry : aField) {
+    ImplCycleCollectionTraverse(
+        aCallback, entry, "TextFrameUnions's key (nsRefPtrHashKey<Element>)",
+        aFlags);
+  }
+}
+
 }  // namespace mozilla::dom
 
 #endif  // mozilla_dom_PerformanceMainThread_h
diff --git a/dom/performance/PerformanceObserver.cpp b/dom/performance/PerformanceObserver.cpp
index e132b70860c2..d5d1725c5773 100644
--- a/dom/performance/PerformanceObserver.cpp
+++ b/dom/performance/PerformanceObserver.cpp
@@ -17,6 +17,7 @@
 #include "nsQueryObject.h"
 #include "nsString.h"
 #include "PerformanceEntry.h"
+#include "LargestContentfulPaint.h"
 #include "PerformanceObserverEntryList.h"
 
 using namespace mozilla;
@@ -215,6 +216,12 @@ void PerformanceObserver::Observe(const PerformanceObserverInit& aOptions,
         }
       }
     }
+    if (StaticPrefs::dom_enable_largest_contentful_paint()) {
+      if (entryTypes.Contains(kLargestContentfulPaintName) &&
+          !validEntryTypes.Contains(kLargestContentfulPaintName)) {
+        validEntryTypes.AppendElement(kLargestContentfulPaintName);
+      }
+    }
     for (const nsLiteralString& name : kValidTypeNames) {
       if (entryTypes.Contains(name) && !validEntryTypes.Contains(name)) {
         validEntryTypes.AppendElement(name);
@@ -275,6 +282,12 @@ void PerformanceObserver::Observe(const PerformanceObserverInit& aOptions,
       }
     }
 
+    if (StaticPrefs::dom_enable_largest_contentful_paint()) {
+      if (type == kLargestContentfulPaintName) {
+        typeValid = true;
+      }
+    }
+
     if (!typeValid) {
       ReportUnsupportedTypesErrorToConsole(
           NS_IsMainThread(), UnsupportedEntryTypesIgnoredMsgId, type);
@@ -328,6 +341,10 @@ void PerformanceObserver::GetSupportedEntryTypes(
       validTypes.AppendElement(name);
     }
   }
+
+  if (StaticPrefs::dom_enable_largest_contentful_paint()) {
+    validTypes.AppendElement(u"largest-contentful-paint"_ns);
+  }
   for (const nsLiteralString& name : kValidTypeNames) {
     validTypes.AppendElement(name);
   }
diff --git a/dom/tests/mochitest/general/test_interfaces.js b/dom/tests/mochitest/general/test_interfaces.js
index 40d418d4b1b6..3edaee8d5595 100644
--- a/dom/tests/mochitest/general/test_interfaces.js
+++ b/dom/tests/mochitest/general/test_interfaces.js
@@ -776,6 +776,8 @@ let interfaceNamesInGlobalScope = [
   // IMPORTANT: Do not change this list without review from a DOM peer!
   { name: "KeyframeEffect", insecureContext: true },
   // IMPORTANT: Do not change this list without review from a DOM peer!
+  { name: "LargestContentfulPaint", insecureContext: true, nightly: true },
+  // IMPORTANT: Do not change this list without review from a DOM peer!
   { name: "Location", insecureContext: true },
   // IMPORTANT: Do not change this list without review from a DOM peer!
   "Lock",
diff --git a/image/imgRequest.cpp b/image/imgRequest.cpp
index 27fd7e29b0d8..47bdd3480cf0 100644
--- a/image/imgRequest.cpp
+++ b/image/imgRequest.cpp
@@ -63,6 +63,7 @@ imgRequest::imgRequest(imgLoader* aLoader, const ImageCacheKey& aCacheKey)
       mImageAvailable(false),
       mIsDeniedCrossSiteCORSRequest(false),
       mIsCrossSiteNoCORSRequest(false),
+      mShouldReportRenderTimeForLCP(false),
       mMutex("imgRequest"),
       mProgressTracker(new ProgressTracker()),
       mIsMultiPartChannel(false),
@@ -641,6 +642,7 @@ imgRequest::OnStartRequest(nsIRequest* aRequest) {
     mIsCrossSiteNoCORSRequest = loadInfo->GetTainting() == LoadTainting::Opaque;
   }
 
+  UpdateShouldReportRenderTimeForLCP();
   // Figure out if we're multipart.
   nsCOMPtr<nsIMultiPartChannel> multiPartChannel = do_QueryInterface(aRequest);
   {
@@ -1232,3 +1234,13 @@ imgRequest::OnRedirectVerifyCallback(nsresult result) {
   mRedirectCallback = nullptr;
   return NS_OK;
 }
+
+void imgRequest::UpdateShouldReportRenderTimeForLCP() {
+  if (mTimedChannel) {
+    bool allRedirectPassTAO = false;
+    mTimedChannel->GetAllRedirectsPassTimingAllowCheck(&allRedirectPassTAO);
+    mShouldReportRenderTimeForLCP =
+        mTimedChannel->TimingAllowCheck(mTriggeringPrincipal) &&
+        allRedirectPassTAO;
+  }
+}
diff --git a/image/imgRequest.h b/image/imgRequest.h
index 7a751e650524..b948869ae81d 100644
--- a/image/imgRequest.h
+++ b/image/imgRequest.h
@@ -202,6 +202,10 @@ class imgRequest final : public nsIThreadRetargetableStreamListener,
 
   bool IsCrossSiteNoCORSRequest() const { return mIsCrossSiteNoCORSRequest; }
 
+  bool ShouldReportRenderTimeForLCP() const {
+    return mShouldReportRenderTimeForLCP;
+  }
+
  private:
   friend class FinishPreparingForNewPartRunnable;
 
@@ -209,6 +213,8 @@ class imgRequest final : public nsIThreadRetargetableStreamListener,
 
   void FinishPreparingForNewPart(const NewPartResult& aResult);
 
+  void UpdateShouldReportRenderTimeForLCP();
+
   void Cancel(nsresult aStatus);
 
   // Update the cache entry size based on the image container.
@@ -276,6 +282,8 @@ class imgRequest final : public nsIThreadRetargetableStreamListener,
   bool mIsDeniedCrossSiteCORSRequest;
   bool mIsCrossSiteNoCORSRequest;
 
+  bool mShouldReportRenderTimeForLCP;
+
   mutable mozilla::Mutex mMutex;
 
   // Member variables protected by mMutex. Note that *all* flags in our bitfield
diff --git a/image/imgRequestProxy.h b/image/imgRequestProxy.h
index 72e5637f7b20..b745ac21b56d 100644
--- a/image/imgRequestProxy.h
+++ b/image/imgRequestProxy.h
@@ -225,6 +225,10 @@ class imgRequestProxy : public mozilla::PreloaderBase,
   bool mHadListener : 1;
 };
 
+inline nsISupports* ToSupports(imgRequestProxy* p) {
+  return NS_ISUPPORTS_CAST(imgIRequest*, p);
+}
+
 NS_DEFINE_STATIC_IID_ACCESSOR(imgRequestProxy, NS_IMGREQUESTPROXY_CID)
 
 // Used for static image proxies for which no requests are available, so
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index b02283dea6f9..f481c6462faf 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -11,6 +11,8 @@
 #include "Units.h"
 #include "mozilla/dom/FontFaceSet.h"
 #include "mozilla/dom/ElementBinding.h"
+#include "mozilla/dom/LargestContentfulPaint.h"
+#include "mozilla/dom/PerformanceMainThread.h"
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/AutoRestore.h"
@@ -11839,6 +11841,13 @@ void PresShell::EndPaint() {
       }
       return CallState::Continue;
     });
+
+    if (nsPresContext* presContext = GetPresContext()) {
+      if (PerformanceMainThread* perf =
+              presContext->GetPerformanceMainThread()) {
+        perf->FinalizeLCPEntriesForText();
+      }
+    }
   }
 }
 
diff --git a/layout/base/PresShell.h b/layout/base/PresShell.h
index cc733aa7ea94..20ebcbc7a3eb 100644
--- a/layout/base/PresShell.h
+++ b/layout/base/PresShell.h
@@ -85,6 +85,7 @@ struct RangePaintInfo;
 class ReflowCountMgr;
 #endif
 class WeakFrame;
+class nsTextFrame;
 class ZoomConstraintsClient;
 
 struct nsCallbackEventRequest;
@@ -113,6 +114,7 @@ class Element;
 class Event;
 class HTMLSlotElement;
 class Selection;
+class PerformanceMainThread;
 }  // namespace dom
 
 namespace gfx {
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 7b5da89a0b06..4b0d5d808731 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -89,6 +89,7 @@
 #include "mozilla/Telemetry.h"
 #include "mozilla/TimelineManager.h"
 #include "mozilla/dom/Performance.h"
+#include "mozilla/dom/PerformanceMainThread.h"
 #include "mozilla/dom/PerformanceTiming.h"
 #include "mozilla/dom/PerformancePaintTiming.h"
 #include "mozilla/layers/APZThreadUtils.h"
@@ -2781,8 +2782,16 @@ void nsPresContext::NotifyNonBlankPaint() {
   }
 }
 
+bool nsPresContext::HasStoppedGeneratingLCP() const {
+  if (auto* perf = GetPerformanceMainThread()) {
+    return perf->HasDispatchedInputEvent() || perf->HasDispatchedScrollEvent();
+  }
+
+  return true;
+}
+
 void nsPresContext::NotifyContentfulPaint() {
-  if (mHadFirstContentfulPaint) {
+  if (mHadFirstContentfulPaint && HasStoppedGeneratingLCP()) {
     return;
   }
   nsRootPresContext* rootPresContext = GetRootPresContext();
@@ -2804,19 +2813,24 @@ void nsPresContext::NotifyContentfulPaint() {
     }
     return;
   }
-  mHadFirstContentfulPaint = true;
-  mFirstContentfulPaintTransactionId =
-      Some(rootPresContext->mRefreshDriver->LastTransactionId().Next());
-  if (nsPIDOMWindowInner* innerWindow = mDocument->GetInnerWindow()) {
-    if (Performance* perf = innerWindow->GetPerformance()) {
+
+  if (!mHadFirstContentfulPaint) {
+    mHadFirstContentfulPaint = true;
+    mFirstContentfulPaintTransactionId =
+        Some(rootPresContext->mRefreshDriver->LastTransactionId().Next());
+  }
+
+  if (auto* perf = GetPerformanceMainThread()) {
+    mMarkPaintTimingStart = TimeStamp::Now();
+    MOZ_ASSERT(rootPresContext->RefreshDriver()->IsInRefresh(),
+               "We should only notify contentful paint during refresh "
+               "driver ticks");
+    if (!perf->HadFCPTimingEntry()) {
       TimeStamp nowTime = rootPresContext->RefreshDriver()->MostRecentRefresh(
           /* aEnsureTimerStarted */ false);
       MOZ_ASSERT(!nowTime.IsNull(),
                  "Most recent refresh timestamp should exist since we are in "
                  "a refresh driver tick");
-      MOZ_ASSERT(rootPresContext->RefreshDriver()->IsInRefresh(),
-                 "We should only notify contentful paint during refresh "
-                 "driver ticks");
       RefPtr<PerformancePaintTiming> paintTiming = new PerformancePaintTiming(
           perf, u"first-contentful-paint"_ns, nowTime);
       perf->SetFCPTimingEntry(paintTiming);
@@ -2833,12 +2847,15 @@ void nsPresContext::NotifyContentfulPaint() {
               nsContentUtils::TruncatedURLForDisplay(docURI).get());
           PROFILER_MARKER_TEXT(
               "FirstContentfulPaint", DOM,
-              MarkerOptions(MarkerTiming::Interval(navigationStart, nowTime),
-                            MarkerInnerWindowId(innerWindow->WindowID())),
+              MarkerOptions(
+                  MarkerTiming::Interval(navigationStart, nowTime),
+                  MarkerInnerWindowId(mDocument->GetInnerWindow()->WindowID())),
               marker);
         }
       }
     }
+
+    perf->ProcessElementTiming();
   }
 }
 
@@ -3050,6 +3067,16 @@ void nsPresContext::SetSafeAreaInsets(const ScreenIntMargin& aSafeAreaInsets) {
                                RestyleHint::RecascadeSubtree());
 }
 
+PerformanceMainThread* nsPresContext::GetPerformanceMainThread() const {
+  if (nsPIDOMWindowInner* innerWindow = mDocument->GetInnerWindow()) {
+    if (auto* perf = static_cast<PerformanceMainThread*>(
+            innerWindow->GetPerformance())) {
+      return perf;
+    }
+  }
+  return nullptr;
+}
+
 #ifdef DEBUG
 
 void nsPresContext::ValidatePresShellAndDocumentReleation() const {
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 3d723066a99c..ed99f9a72026 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -92,6 +92,7 @@ class LayerManager;
 namespace dom {
 class Document;
 class Element;
+class PerformanceMainThread;
 enum class PrefersColorSchemeOverride : uint8_t;
 }  // namespace dom
 namespace gfx {
@@ -214,6 +215,7 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
 
   void DocumentCharSetChanged(NotNull<const Encoding*> aCharSet);
 
+  mozilla::dom::PerformanceMainThread* GetPerformanceMainThread() const;
   /**
    * Returns the parent prescontext for this one. Returns null if this is a
    * root.
@@ -1040,6 +1042,7 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
 
   bool HadNonBlankPaint() const { return mHadNonBlankPaint; }
   bool HadFirstContentfulPaint() const { return mHadFirstContentfulPaint; }
+  bool HasStoppedGeneratingLCP() const;
   void NotifyNonBlankPaint();
   void NotifyContentfulPaint();
   void NotifyPaintStatusReset();
@@ -1122,6 +1125,10 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
     }
   }
 
+  mozilla::TimeStamp GetMarkPaintTimingStart() const {
+    return mMarkPaintTimingStart;
+  }
+
  protected:
   // May be called multiple times (unlink, destructor)
   void Destroy();
@@ -1245,6 +1252,9 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
 
   mozilla::TimeStamp mReflowStartTime;
 
+  // Defined in https://w3c.github.io/paint-timing/#mark-paint-timing step 2.
+  mozilla::TimeStamp mMarkPaintTimingStart;
+
   Maybe<TransactionId> mFirstContentfulPaintTransactionId;
 
   mozilla::UniquePtr<mozilla::MediaFeatureChange>
diff --git a/layout/base/nsRefreshDriver.cpp b/layout/base/nsRefreshDriver.cpp
index 2c4689b22aae..5f881fde6052 100644
--- a/layout/base/nsRefreshDriver.cpp
+++ b/layout/base/nsRefreshDriver.cpp
@@ -45,6 +45,7 @@
 #include "nsITimer.h"
 #include "nsLayoutUtils.h"
 #include "nsPresContext.h"
+#include "imgRequest.h"
 #include "nsComponentManagerUtils.h"
 #include "mozilla/Logging.h"
 #include "mozilla/dom/Document.h"
@@ -52,6 +53,7 @@
 #include "nsIXULRuntime.h"
 #include "jsapi.h"
 #include "nsContentUtils.h"
+#include "nsTextFrame.h"
 #include "mozilla/PendingAnimationTracker.h"
 #include "mozilla/PendingFullscreenEvent.h"
 #include "mozilla/dom/PerformanceMainThread.h"
@@ -71,6 +73,7 @@
 #include "mozilla/dom/Selection.h"
 #include "mozilla/dom/VsyncMainChild.h"
 #include "mozilla/dom/WindowBinding.h"
+#include "mozilla/dom/LargestContentfulPaint.h"
 #include "mozilla/layers/WebRenderLayerManager.h"
 #include "mozilla/RestyleManager.h"
 #include "mozilla/TaskController.h"
diff --git a/layout/generic/moz.build b/layout/generic/moz.build
index cfb84e4864f2..e76de4becc93 100644
--- a/layout/generic/moz.build
+++ b/layout/generic/moz.build
@@ -143,6 +143,7 @@ EXPORTS.mozilla += [
     "CSSAlignUtils.h",
     "CSSOrderAwareFrameIterator.h",
     "LayoutMessageUtils.h",
+    "nsVideoFrame.h",
     "PrintedSheetFrame.h",
     "ReflowInput.h",
     "ReflowOutput.h",
diff --git a/layout/generic/nsIFrame.h b/layout/generic/nsIFrame.h
index 80c3504e1af3..bbf3db4f70a3 100644
--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -5582,6 +5582,8 @@ class MOZ_HEAP_CLASS WeakFrame {
   nsIFrame* operator->() { return mFrame; }
   operator nsIFrame*() { return mFrame; }
 
+  bool operator==(nsIFrame* const aOther) const { return mFrame == aOther; }
+
   void Clear(mozilla::PresShell* aPresShell);
 
   bool IsAlive() const { return !!mFrame; }
diff --git a/layout/generic/nsImageFrame.cpp b/layout/generic/nsImageFrame.cpp
index 4b3d9184e54f..f158f118f8dd 100644
--- a/layout/generic/nsImageFrame.cpp
+++ b/layout/generic/nsImageFrame.cpp
@@ -27,6 +27,7 @@
 #include "mozilla/dom/HTMLImageElement.h"
 #include "mozilla/dom/ReferrerInfo.h"
 #include "mozilla/dom/ResponsiveImageSelector.h"
+#include "mozilla/dom/LargestContentfulPaint.h"
 #include "mozilla/image/WebRenderImageProvider.h"
 #include "mozilla/layers/RenderRootStateManager.h"
 #include "mozilla/layers/WebRenderLayerManager.h"
@@ -1115,6 +1116,8 @@ void nsImageFrame::Notify(imgIRequest* aRequest, int32_t aType,
   }
 
   if (aType == imgINotificationObserver::LOAD_COMPLETE) {
+    LargestContentfulPaint::MaybeProcessImageForElementTiming(
+        static_cast<imgRequestProxy*>(aRequest), GetContent()->AsElement());
     uint32_t imgStatus;
     aRequest->GetImageStatus(&imgStatus);
     nsresult status =
@@ -2365,6 +2368,13 @@ bool nsDisplayImage::CreateWebRenderCommands(
       mImage->GetImageProvider(aManager->LayerManager(), decodeSize, svgContext,
                                region, flags, getter_AddRefs(provider));
 
+  if (nsCOMPtr<imgIRequest> currentRequest = frame->GetCurrentRequest()) {
+    LCPHelpers::FinalizeLCPEntryForImage(
+        frame->GetContent()->AsElement(),
+        static_cast<imgRequestProxy*>(currentRequest.get()),
+        GetDestRect() - ToReferenceFrame());
+  }
+
   // While we got a container, it may not contain a fully decoded surface. If
   // that is the case, and we have an image we were previously displaying which
   // has a fully decoded surface, then we should prefer the previous image.
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index b0e4f61119ed..db1b2e70aaf3 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -27,6 +27,7 @@
 #include "mozilla/IntegerRange.h"
 #include "mozilla/Unused.h"
 #include "mozilla/PodOperations.h"
+#include "mozilla/dom/PerformanceMainThread.h"
 
 #include "nsCOMPtr.h"
 #include "nsBlockFrame.h"
diff --git a/layout/painting/nsDisplayList.cpp b/layout/painting/nsDisplayList.cpp
index 5357333a1875..6c0f119cbe94 100644
--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -28,6 +28,7 @@
 #include "mozilla/dom/ServiceWorkerRegistration.h"
 #include "mozilla/dom/SVGElement.h"
 #include "mozilla/dom/TouchEvent.h"
+#include "mozilla/dom/PerformanceMainThread.h"
 #include "mozilla/gfx/2D.h"
 #include "mozilla/PresShell.h"
 #include "mozilla/ShapeUtils.h"
@@ -1220,7 +1221,7 @@ void nsDisplayListBuilder::LeavePresShell(const nsIFrame* aReferenceFrame,
       }
     }
     nsRootPresContext* rootPresContext = pc->GetRootPresContext();
-    if (!pc->HadFirstContentfulPaint() && rootPresContext) {
+    if (!pc->HasStoppedGeneratingLCP() && rootPresContext) {
       if (!CurrentPresShellState()->mIsBackgroundOnly) {
         if (pc->HasEverBuiltInvisibleText() ||
             DisplayListIsContentful(this, aPaintedContents)) {
@@ -3429,6 +3430,17 @@ bool nsDisplayBackgroundImage::CreateWebRenderCommands(
   if (result == ImgDrawResult::NOT_SUPPORTED) {
     return false;
   }
+
+  if (nsIContent* content = StyleFrame()->GetContent()) {
+    if (imgRequestProxy* requestProxy = mBackgroundStyle->StyleBackground()
+                                            ->mImage.mLayers[mLayer]
+                                            .mImage.GetImageRequest()) {
+      // LCP don't consider gradient backgrounds.
+      LCPHelpers::FinalizeLCPEntryForImage(content->AsElement(), requestProxy,
+                                           mBounds - ToReferenceFrame());
+    }
+  }
+
   return true;
 }
 
@@ -7483,6 +7495,10 @@ void nsDisplayText::Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) {
   // the WebRender fallback painting path, and we don't want to issue
   // recorded commands that are dependent on the visible/building rect.
   RenderToContext(aCtx, aBuilder, GetPaintRect(aBuilder, aCtx));
+
+  auto* textFrame = static_cast<nsTextFrame*>(mFrame);
+  LCPTextFrameHelper::MaybeUnionTextFrame(textFrame,
+                                          mBounds - ToReferenceFrame());
 }
 
 bool nsDisplayText::CreateWebRenderCommands(
@@ -7568,6 +7584,8 @@ bool nsDisplayText::CreateWebRenderCommands(
   gfxContext* textDrawer = aBuilder.GetTextContext(aResources, aSc, aManager,
                                                    this, bounds, deviceOffset);
 
+  LCPTextFrameHelper::MaybeUnionTextFrame(f, bounds - ToReferenceFrame());
+
   aBuilder.StartGroup(this);
 
   RenderToContext(textDrawer, aDisplayListBuilder, mVisibleRect,
diff --git a/layout/style/ImageLoader.cpp b/layout/style/ImageLoader.cpp
index c60f539751d0..c56ecdfd20c1 100644
--- a/layout/style/ImageLoader.cpp
+++ b/layout/style/ImageLoader.cpp
@@ -12,6 +12,7 @@
 
 #include "mozilla/dom/Document.h"
 #include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/dom/LargestContentfulPaint.h"
 #include "mozilla/dom/ImageTracker.h"
 #include "nsContentUtils.h"
 #include "nsIReflowCallback.h"
@@ -818,10 +819,16 @@ void ImageLoader::OnLoadComplete(imgIRequest* aRequest) {
       // may happen during other network events.
       UnblockOnloadIfNeeded(fwf);
     }
-    if (fwf.mFrame->StyleVisibility()->IsVisible()) {
-      fwf.mFrame->SchedulePaint();
+    nsIFrame* frame = fwf.mFrame;
+    if (frame->StyleVisibility()->IsVisible()) {
+      frame->SchedulePaint();
+    }
+
+    if (StaticPrefs::dom_enable_largest_contentful_paint()) {
+      LargestContentfulPaint::MaybeProcessImageForElementTiming(
+          static_cast<imgRequestProxy*>(aRequest),
+          frame->GetContent()->AsElement());
     }
   }
 }
-
 }  // namespace mozilla::css
diff --git a/layout/svg/SVGImageFrame.cpp b/layout/svg/SVGImageFrame.cpp
index da04beee71bd..ae1cf1ded7d1 100644
--- a/layout/svg/SVGImageFrame.cpp
+++ b/layout/svg/SVGImageFrame.cpp
@@ -28,6 +28,7 @@
 #include "mozilla/SVGUtils.h"
 #include "mozilla/dom/MutationEventBinding.h"
 #include "mozilla/dom/SVGImageElement.h"
+#include "mozilla/dom/LargestContentfulPaint.h"
 #include "nsIReflowCallback.h"
 
 using namespace mozilla::dom;
@@ -391,6 +392,12 @@ void SVGImageFrame::PaintSVG(gfxContext& aContext, const gfxMatrix& aTransform,
       LayoutDeviceSize devPxSize(width, height);
       nsRect destRect(nsPoint(), LayoutDevicePixel::ToAppUnits(
                                      devPxSize, appUnitsPerDevPx));
+      nsCOMPtr<imgIRequest> currentRequest = GetCurrentRequest();
+      if (currentRequest) {
+        LCPHelpers::FinalizeLCPEntryForImage(
+            GetContent()->AsElement(),
+            static_cast<imgRequestProxy*>(currentRequest.get()), destRect);
+      }
 
       // Note: Can't use DrawSingleUnscaledImage for the TYPE_VECTOR case.
       // That method needs our image to have a fixed native width & height,
@@ -467,14 +474,7 @@ bool SVGImageFrame::CreateWebRenderCommands(
 
   // try to setup the image
   if (!mImageContainer) {
-    nsCOMPtr<imgIRequest> currentRequest;
-    nsCOMPtr<nsIImageLoadingContent> imageLoader =
-        do_QueryInterface(GetContent());
-    if (imageLoader) {
-      imageLoader->GetRequest(nsIImageLoadingContent::CURRENT_REQUEST,
-                              getter_AddRefs(currentRequest));
-    }
-
+    nsCOMPtr<imgIRequest> currentRequest = GetCurrentRequest();
     if (currentRequest) {
       currentRequest->GetImage(getter_AddRefs(mImageContainer));
     }
@@ -633,6 +633,14 @@ bool SVGImageFrame::CreateWebRenderCommands(
       mImageContainer, this, destRect, clipRect, aSc, flags, svgContext,
       region);
 
+  if (nsCOMPtr<imgIRequest> currentRequest = GetCurrentRequest()) {
+    LCPHelpers::FinalizeLCPEntryForImage(
+        GetContent()->AsElement(),
+        static_cast<imgRequestProxy*>(currentRequest.get()),
+        LayoutDeviceRect::ToAppUnits(destRect, appUnitsPerDevPx) -
+            toReferenceFrame);
+  }
+
   RefPtr<image::WebRenderImageProvider> provider;
   ImgDrawResult drawResult = mImageContainer->GetImageProvider(
       aManager->LayerManager(), decodeSize, svgContext, region, flags,
@@ -783,6 +791,17 @@ bool SVGImageFrame::ReflowFinished() {
 
 void SVGImageFrame::ReflowCallbackCanceled() { mReflowCallbackPosted = false; }
 
+already_AddRefed<imgIRequest> SVGImageFrame::GetCurrentRequest() const {
+  nsCOMPtr<imgIRequest> request;
+  nsCOMPtr<nsIImageLoadingContent> imageLoader =
+      do_QueryInterface(GetContent());
+  if (imageLoader) {
+    imageLoader->GetRequest(nsIImageLoadingContent::CURRENT_REQUEST,
+                            getter_AddRefs(request));
+  }
+  return request.forget();
+}
+
 bool SVGImageFrame::IgnoreHitTest() const {
   switch (Style()->PointerEvents()) {
     case StylePointerEvents::None:
diff --git a/layout/svg/SVGImageFrame.h b/layout/svg/SVGImageFrame.h
index 71e291e57953..7f8cbde781e0 100644
--- a/layout/svg/SVGImageFrame.h
+++ b/layout/svg/SVGImageFrame.h
@@ -118,6 +118,8 @@ class SVGImageFrame final : public nsIFrame,
  private:
   bool IgnoreHitTest() const;
 
+  already_AddRefed<imgIRequest> GetCurrentRequest() const;
+
   gfx::Matrix GetRasterImageTransform(int32_t aNativeWidth,
                                       int32_t aNativeHeight);
   gfx::Matrix GetVectorImageTransform();
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index 006c52e493bf..63004725cb97 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -2392,7 +2392,7 @@
 # Whether the LargestContentfulPaint API will be gathered and returned by performance observer*
 - name: dom.enable_largest_contentful_paint
   type: RelaxedAtomicBool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 # Whether performance.GetEntries* will contain an entry for the active document