--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -46,16 +46,19 @@
 #include "nsDOMCSSDeclaration.h"
 #include "nsStyleTransformMatrix.h"
 #include "mozilla/dom/Element.h"
 #include "prtime.h"
 #include "nsWrapperCacheInlines.h"
 #include "mozilla/AppUnits.h"
 #include <algorithm>
 
+#include "mozilla/CSSComputedValue.h"
+#include "mozilla/CSSVariableValues.h"
+
 using namespace mozilla;
 using namespace mozilla::dom;
 
 #if defined(DEBUG_bzbarsky) || defined(DEBUG_caillon)
 #define DEBUG_ComputedDOMStyle
 #endif
 
 /*
@@ -6600,28 +6603,33 @@ MarkComputedStyleMapDirty(const char* aP
   static_cast<nsComputedStyleMap*>(aData)->MarkDirty();
 }
 
 already_AddRefed<CSSValue>
 nsComputedDOMStyle::DoGetCustomProperty(const nsAString& aPropertyName)
 {
   MOZ_ASSERT(nsCSSProps::IsCustomPropertyName(aPropertyName));
 
-  const nsStyleVariables* variables = StyleVariables();
-
-  nsString variableValue;
   const nsAString& name = Substring(aPropertyName,
                                     CSS_CUSTOM_NAME_PREFIX_LENGTH);
-  if (!variables->mVariables.Get(name, variableValue)) {
+
+  const CSSVariableValues& variables = StyleVariables()->mVariables;
+  if (!variables.Has(name)) {
     return nullptr;
   }
 
-  RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
-  val->SetString(variableValue);
-
+  nsString stringValue;
+  const CSSComputedValue* value;
+  if (variables.Compute(name, &value)) {
+    value->AppendToString(stringValue);
+  } else {
+    variables.Get(name, stringValue);
+  }
+  RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
+  val->SetString(stringValue);
   return val.forget();
 }
 
 void
 nsComputedDOMStyle::ParentChainChanged(nsIContent* aContent)
 {
   NS_ASSERTION(mContent == aContent, "didn't we register mContent?");
   NS_ASSERTION(mResolvedStyleContext,