--- a/dom/base/AnonymousContent.cpp
+++ b/dom/base/AnonymousContent.cpp
@@ -2,16 +2,17 @@
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "AnonymousContent.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/AnonymousContentBinding.h"
+#include "nsComputedDOMStyle.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsIDocument.h"
 #include "nsIDOMHTMLCollection.h"
 #include "nsIFrame.h"
 #include "nsStyledElement.h"
 #include "HTMLCanvasElement.h"
 
 namespace mozilla {
@@ -203,10 +204,34 @@ AnonymousContent::GetElementById(const n
 bool
 AnonymousContent::WrapObject(JSContext* aCx,
                              JS::Handle<JSObject*> aGivenProto,
                              JS::MutableHandle<JSObject*> aReflector)
 {
   return AnonymousContentBinding::Wrap(aCx, this, aGivenProto, aReflector);
 }
 
+void
+AnonymousContent::GetComputedStylePropertyValue(const nsAString& aElementId,
+                                                const nsAString& aPropertyName,
+                                                DOMString& aResult,
+                                                ErrorResult& aRv)
+{
+  Element* element = GetElementById(aElementId);
+  if (!element) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  nsIPresShell* shell = element->OwnerDoc()->GetShell();
+  if (!shell) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  RefPtr<nsComputedDOMStyle> cs =
+    new nsComputedDOMStyle(element, NS_LITERAL_STRING(""), shell,
+                           nsComputedDOMStyle::eAll);
+  aRv = cs->GetPropertyValue(aPropertyName, aResult);
+}
+
 } // namespace dom
 } // namespace mozilla

--- a/dom/base/AnonymousContent.h
+++ b/dom/base/AnonymousContent.h
@@ -63,16 +63,21 @@ public:
                                                      JS::Handle<JSObject*> aKeyframes,
                                                      const UnrestrictedDoubleOrKeyframeAnimationOptions& aOptions,
                                                      ErrorResult& aError);
 
   void SetCutoutRectsForElement(const nsAString& aElementId,
                                 const Sequence<OwningNonNull<DOMRect>>& aRects,
                                 ErrorResult& aError);
 
+  void GetComputedStylePropertyValue(const nsAString& aElementId,
+                                     const nsAString& aPropertyName,
+                                     DOMString& aResult,
+                                     ErrorResult& aRv);
+
 private:
   ~AnonymousContent();
   nsCOMPtr<Element> mContentNode;
 };
 
 } // namespace dom
 } // namespace mozilla
 

--- a/dom/webidl/AnonymousContent.webidl
+++ b/dom/webidl/AnonymousContent.webidl
@@ -72,9 +72,17 @@ interface AnonymousContent {
    * Accepts a list of (possibly overlapping) DOMRects which describe a shape
    * in CSS pixels relative to the element's border box. This shape will be
    * excluded from the element's background color rendering. The element will
    * not render any background images once this method has been called.
    */
   [Throws]
   void setCutoutRectsForElement(DOMString elementId,
                                 sequence<DOMRect> rects);
+
+  /**
+   * Get the computed value of a property on an element inside this custom
+   * anonymous content.
+   */
+  [Throws]
+  DOMString? getComputedStylePropertyValue(DOMString elementId,
+                                           DOMString propertyName);
 };