diff --git a/dom/base/ResizeObserver.cpp b/dom/base/ResizeObserver.cpp
index 51fd603217..ca25c06e43 100644
--- a/dom/base/ResizeObserver.cpp
+++ b/dom/base/ResizeObserver.cpp
@@ -303,6 +303,30 @@ ResizeObserverEntry::Constructor(const GlobalObject& aGlobal,
   return observerEntry.forget();
 }
 
+void ResizeObserverEntry::GetBorderBoxSize(
+    nsTArray<RefPtr<ResizeObserverSize>>& aRetVal) const {
+  // In the resize-observer-1 spec, there will only be a single
+  // ResizeObserverSize returned in the FrozenArray.
+  //
+  // Note: the usage of FrozenArray is to support elements that have multiple
+  // fragments, which occur in multi-column scenarios. See:
+  // https://drafts.csswg.org/resize-observer/#resize-observer-entry-interface
+  aRetVal.Clear();
+  aRetVal.AppendElement(mBorderBoxSize);
+}
+
+void ResizeObserverEntry::GetContentBoxSize(
+    nsTArray<RefPtr<ResizeObserverSize>>& aRetVal) const {
+  // In the resize-observer-1 spec, there will only be a single
+  // ResizeObserverSize returned in the FrozenArray.
+  //
+  // Note: the usage of FrozenArray is to support elements that have multiple
+  // fragments, which occur in multi-column scenarios.
+  // https://drafts.csswg.org/resize-observer/#resize-observer-entry-interface
+  aRetVal.Clear();
+  aRetVal.AppendElement(mContentBoxSize);
+}
+
 void
 ResizeObserverEntry::SetBorderBoxSize(const nsSize& aSize) {
   nsIFrame* frame = mTarget->GetPrimaryFrame();
diff --git a/dom/base/ResizeObserver.h b/dom/base/ResizeObserver.h
index 56675693ce..ca87a080b4 100644
--- a/dom/base/ResizeObserver.h
+++ b/dom/base/ResizeObserver.h
@@ -163,14 +163,10 @@ public:
 
   /**
    * Returns target's logical border-box size and content-box size as
-   * ResizeObserverSize.
+   * a ResizeObserverSize array.
    */
-  ResizeObserverSize* BorderBoxSize() const { 
-    return mBorderBoxSize;
-  }
-  ResizeObserverSize* ContentBoxSize() const { 
-    return mContentBoxSize;
-  }
+  void GetBorderBoxSize(nsTArray<RefPtr<ResizeObserverSize>>& aRetVal) const;
+  void GetContentBoxSize(nsTArray<RefPtr<ResizeObserverSize>>& aRetVal) const;
 
   // Set borderBoxSize.
   void SetBorderBoxSize(const nsSize& aSize);
diff --git a/dom/webidl/MediaQueryListEvent.webidl b/dom/webidl/MediaQueryListEvent.webidl
index f3a66fb57f..5e53e9734f 100644
--- a/dom/webidl/MediaQueryListEvent.webidl
+++ b/dom/webidl/MediaQueryListEvent.webidl
@@ -1,18 +1,18 @@
-/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this file,
- * You can obtain one at http://mozilla.org/MPL/2.0/.
- *
- * https://drafts.csswg.org/cssom-view/#mediaquerylistevent
- */
-
-[Constructor(DOMString type, optional MediaQueryListEventInit eventInitDict)]
-interface MediaQueryListEvent : Event {
-  readonly attribute DOMString media;
-  readonly attribute boolean matches;
-};
-
-dictionary MediaQueryListEventInit : EventInit {
-  DOMString media = "";
-  boolean matches = false;
-};
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * https://drafts.csswg.org/cssom-view/#mediaquerylistevent
+ */
+
+[Constructor(DOMString type, optional MediaQueryListEventInit eventInitDict)]
+interface MediaQueryListEvent : Event {
+  readonly attribute DOMString media;
+  readonly attribute boolean matches;
+};
+
+dictionary MediaQueryListEventInit : EventInit {
+  DOMString media = "";
+  boolean matches = false;
+};
diff --git a/dom/webidl/ResizeObserver.webidl b/dom/webidl/ResizeObserver.webidl
index d764af7cbd..183bf42304 100644
--- a/dom/webidl/ResizeObserver.webidl
+++ b/dom/webidl/ResizeObserver.webidl
@@ -1,52 +1,56 @@
-/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this file,
- * You can obtain one at http://mozilla.org/MPL/2.0/.
- *
- * The origin of this IDL file is
- * https://wicg.github.io/ResizeObserver/
- */
-
-enum ResizeObserverBoxOptions {
-    "border-box",
-    "content-box"
-};
-
-dictionary ResizeObserverOptions {
-    ResizeObserverBoxOptions box = "content-box";
-};
-
-[Constructor(ResizeObserverCallback callback),
- Exposed=Window,
- Pref="layout.css.resizeobserver.enabled"]
-interface ResizeObserver {
-    [Throws]
-    void observe(Element? target, optional ResizeObserverOptions options);
-    [Throws]
-    void unobserve(Element? target);
-    void disconnect();
-};
-
-callback ResizeObserverCallback = void (sequence<ResizeObserverEntry> entries, ResizeObserver observer);
-
-[Constructor(Element? target),
- Pref="layout.css.resizeobserver.enabled"]
-interface ResizeObserverEntry {
-    readonly attribute Element target;
-    readonly attribute DOMRectReadOnly? contentRect;
-    readonly attribute ResizeObserverSize borderBoxSize;
-    readonly attribute ResizeObserverSize contentBoxSize;
-};
-
-[Pref="layout.css.resizeobserver.enabled"]
-interface ResizeObserverSize {
-    readonly attribute unrestricted double inlineSize;
-    readonly attribute unrestricted double blockSize;
-};
-
-[ChromeOnly,
- Pref="layout.css.resizeobserver.enabled"]
-interface ResizeObservation {
-    readonly attribute Element target;
-    boolean isActive();
-};
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * The origin of this IDL file is
+ * https://wicg.github.io/ResizeObserver/
+ */
+
+enum ResizeObserverBoxOptions {
+    "border-box",
+    "content-box"
+};
+
+dictionary ResizeObserverOptions {
+    ResizeObserverBoxOptions box = "content-box";
+};
+
+[Constructor(ResizeObserverCallback callback),
+ Exposed=Window,
+ Pref="layout.css.resizeobserver.enabled"]
+interface ResizeObserver {
+    [Throws]
+    void observe(Element? target, optional ResizeObserverOptions options);
+    [Throws]
+    void unobserve(Element? target);
+    void disconnect();
+};
+
+callback ResizeObserverCallback = void (sequence<ResizeObserverEntry> entries, ResizeObserver observer);
+
+[Constructor(Element? target),
+ Pref="layout.css.resizeobserver.enabled"]
+interface ResizeObserverEntry {
+    readonly attribute Element target;
+    readonly attribute DOMRectReadOnly? contentRect;
+    // We are using a [Pure, Cached, Frozen] sequence since `FrozenArray` is not implemented in webidl.
+    // This is functionally similar enough. As of #2340 Mozilla has not implemented this yet, either.
+    [Frozen, Cached, Pure]
+    readonly attribute sequence<ResizeObserverSize> borderBoxSize;
+    [Frozen, Cached, Pure]
+    readonly attribute sequence<ResizeObserverSize> contentBoxSize;
+};
+
+[Pref="layout.css.resizeobserver.enabled"]
+interface ResizeObserverSize {
+    readonly attribute unrestricted double inlineSize;
+    readonly attribute unrestricted double blockSize;
+};
+
+[ChromeOnly,
+ Pref="layout.css.resizeobserver.enabled"]
+interface ResizeObservation {
+    readonly attribute Element target;
+    boolean isActive();
+};