diff --git a/dom/base/ResizeObserver.cpp b/dom/base/ResizeObserver.cpp
index 30d029b99aba3..7a47c9541448a 100644
--- a/dom/base/ResizeObserver.cpp
+++ b/dom/base/ResizeObserver.cpp
@@ -168,7 +168,13 @@ ResizeObservation::ResizeObservation(Element& aTarget,
                                      ResizeObserver& aObserver,
                                      ResizeObserverBoxOptions aBox,
                                      WritingMode aWm)
-    : mTarget(&aTarget), mObserver(&aObserver), mObservedBox(aBox) {
+    : mTarget(&aTarget),
+      mObserver(&aObserver),
+      mObservedBox(aBox),
+      mLastReportedSize(
+          aWm, StaticPrefs::dom_resize_observer_last_reported_size_invalid()
+                   ? gfx::Size(-1, -1)
+                   : gfx::Size()) {
   aTarget.BindObject(mObserver);
 }
 
@@ -295,7 +301,8 @@ void ResizeObserver::Observe(Element& aTarget,
   observation =
       new ResizeObservation(aTarget, *this, aOptions.mBox,
                             frame ? frame->GetWritingMode() : WritingMode());
-  if (this == mDocument->GetLastRememberedSizeObserver()) {
+  if (!StaticPrefs::dom_resize_observer_last_reported_size_invalid() &&
+      this == mDocument->GetLastRememberedSizeObserver()) {
     // Resize observations are initialized with a (0, 0) mLastReportedSize,
     // this means that the callback won't be called if the element is 0x0.
     // But we need it called for handling the last remembered size, so set
diff --git a/dom/base/ResizeObserver.h b/dom/base/ResizeObserver.h
index f9e07d68fc33a..c83bfb0b96fa8 100644
--- a/dom/base/ResizeObserver.h
+++ b/dom/base/ResizeObserver.h
@@ -105,8 +105,6 @@ class ResizeObservation final : public LinkedListElement<ResizeObservation> {
   // The latest recorded of observed target.
   // This will be CSS pixels for border-box/content-box, or device pixels for
   // device-pixel-content-box.
-  // Note: We use default constructor for this because we want to start with a
-  // (0, 0) size, per the spec.
   LogicalPixelSize mLastReportedSize;
 };
 
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index 36d0c87ce32a3..79ab2d5bb008d 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -4581,6 +4581,13 @@
   value: 5
   mirror: always
 
+# Initialize Resize Observer's last reported size to -1x-1, and not 0x0,
+# as per CSSWG resolution: https://github.com/w3c/csswg-drafts/issues/3664
+- name: dom.resize_observer.last_reported_size_invalid
+  type: bool
+  value: true
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "editor"
 #---------------------------------------------------------------------------
diff --git a/testing/web-platform/tests/resize-observer/notify.html b/testing/web-platform/tests/resize-observer/notify.html
index d5888515d5a08..7f0e448540465 100644
--- a/testing/web-platform/tests/resize-observer/notify.html
+++ b/testing/web-platform/tests/resize-observer/notify.html
@@ -194,27 +194,38 @@
 
 function test6() {
   let helper = new ResizeTestHelper(
-    "test6: inline element does not notify",
+    "test6: inline element notifies once with 0x0.",
   [
     {
       setup: observer => {
         observer.observe(inline);
-        observer.observe(t1);
-        t1.style.width = "66px";
-        inline.style.width = "66px";
       },
       notify: (entries, observer) => {
-        assert_equals(entries.length, 1, "inline elements must not trigger notifications");
-        assert_equals(entries[0].target, t1, "inline elements must not trigger notifications");
+        assert_equals(entries.length, 1, "observing inline element triggers notification");
+        assert_equals(entries[0].target, inline, "observing inline element triggers notification");
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
         return true; // Delay next step
       }
     },
+    {
+      setup: observer => {
+        inline.style.width = "66px";
+      },
+      notify: (entries, observer) => {
+        assert_unreached("resizing inline element should not cause resize notifications");
+      },
+      timeout: () => {
+        // expected
+      }
+    },
     { // "inline element that becomes block should notify",
       setup: observer => {
         inline.style.display = "block";
       },
       notify: (entries, observer) => {
-        assert_equals(entries[0].target, inline);
+        assert_equals(entries.length, 1, "inline element becoming a non-zero sized block triggers a notification");
+        assert_equals(entries[0].target, inline, "inline element becoming a non-zero sized block triggers a notification");
       }
     }
   ]);
diff --git a/testing/web-platform/tests/resize-observer/observe.html b/testing/web-platform/tests/resize-observer/observe.html
index a3bf784b62cfc..f6015bef78825 100644
--- a/testing/web-platform/tests/resize-observer/observe.html
+++ b/testing/web-platform/tests/resize-observer/observe.html
@@ -719,16 +719,26 @@
 
   let helper = new ResizeTestHelper(
     // See: https://drafts.csswg.org/resize-observer/#intro.
-    "test16: observations do not fire for non-replaced inline elements",
+    "test16: observations fire once with 0x0 size for non-replaced inline elements",
   [
     {
       setup: observer => {
         observer.observe(t);
       },
       notify: entries => {
-        assert_unreached("No observation should fire for non box element")
-      },
-      timeout: () => {}
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 0, "target width");
+        assert_equals(entries[0].contentRect.height, 0, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 0,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 0,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 0,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 0,
+                      "target border-box block size");
+      }
     }
   ]);
 
diff --git a/testing/web-platform/tests/resize-observer/svg.html b/testing/web-platform/tests/resize-observer/svg.html
index 08c1cf9a37349..6511afc8b000a 100644
--- a/testing/web-platform/tests/resize-observer/svg.html
+++ b/testing/web-platform/tests/resize-observer/svg.html
@@ -345,10 +345,10 @@
         observer.observe(view);
       },
       notify: (entries, observer) => {
-        assert_unreached("no entries should be observed on <view> Element");
-      },
-      timeout: () => {
-        // expected
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].target, view);
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
       }
     },
     {
@@ -356,10 +356,10 @@
         observer.observe(stop);
       },
       notify: (entries, observer) => {
-        assert_unreached("no entries should be observed on <stop> Element");
-      },
-      timeout: () => {
-        // expected
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].target, stop);
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
       }
     },
   ]);