--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -1848,16 +1848,29 @@ nsPresContext::MediaFeatureValuesChanged
       aChangeHint
     };
 
     mDocument->EnumerateSubDocuments(MediaFeatureValuesChangedAllDocumentsCallback,
                                      &hints);
 }
 
 void
+nsPresContext::RefreshRuleCascade()
+{
+  if (mShell) {
+    if (mShell->StyleSet()->IsGecko()) {
+      mShell->StyleSet()->AsGecko()->RefreshRuleCascade();
+    } else {
+      NS_WARNING("stylo: ServoStyleSet needs to expose the same functionality. "
+                 "See also the warning in MediaFeatureValuesChanged.");
+    }
+  }
+}
+
+void
 nsPresContext::MediaFeatureValuesChanged(nsRestyleHint aRestyleHint,
                                          nsChangeHint aChangeHint)
 {
   mPendingMediaFeatureValuesChanged = false;
 
   // MediumFeaturesChanged updates the applied rules, so it always gets called.
   if (mShell) {
     // XXXheycam ServoStyleSets don't support responding to medium

--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -262,16 +262,22 @@ public:
   /**
    * Just like RebuildAllStyleData, except (1) asynchronous and (2) it
    * doesn't rebuild the user font set.
    */
   void PostRebuildAllStyleDataEvent(nsChangeHint aExtraHint,
                                     nsRestyleHint aRestyleHint);
 
   /**
+   * Refresh the rule cascade. This does not rebuild all style data -- call
+   * RebuildAllStyleData that is necessary.
+   */
+  void RefreshRuleCascade();
+
+  /**
    * Handle changes in the values of media features (used in media
    * queries).
    *
    * There are three sensible values to use for aRestyleHint:
    *  * nsRestyleHint(0) to rebuild style data, with rerunning of
    *    selector matching, only if media features have changed
    *  * eRestyle_ForceDescendants to force rebuilding of style data (but
    *    still only rerun selector matching if media query results have

--- a/layout/style/nsCSSRuleProcessor.cpp
+++ b/layout/style/nsCSSRuleProcessor.cpp
@@ -3756,17 +3756,17 @@ nsCSSRuleProcessor::GetRuleCascade(nsPre
   if (!mRuleCascades || aPresContext != mLastPresContext) {
     RefreshRuleCascade(aPresContext);
   }
   mLastPresContext = aPresContext;
 
   return mRuleCascades;
 }
 
-void
+/* virtual */ void
 nsCSSRuleProcessor::RefreshRuleCascade(nsPresContext* aPresContext)
 {
   // Having RuleCascadeData objects be per-medium (over all variation
   // caused by media queries, handled through mCacheKey) works for now
   // since nsCSSRuleProcessor objects are per-document.  (For a given
   // set of stylesheets they can vary based on medium (@media) or
   // document (@-moz-document).)
 

--- a/layout/style/nsCSSRuleProcessor.h
+++ b/layout/style/nsCSSRuleProcessor.h
@@ -229,17 +229,17 @@ public:
 protected:
   virtual ~nsCSSRuleProcessor();
 
 private:
   static bool CascadeSheet(mozilla::CSSStyleSheet* aSheet,
                            CascadeEnumData* aData);
 
   RuleCascadeData* GetRuleCascade(nsPresContext* aPresContext);
-  void RefreshRuleCascade(nsPresContext* aPresContext);
+  virtual void RefreshRuleCascade(nsPresContext* aPresContext) override;
 
   nsRestyleHint HasStateDependentStyle(ElementDependentRuleProcessorData* aData,
                                        mozilla::dom::Element* aStatefulElement,
                                        mozilla::CSSPseudoElementType aPseudoType,
                                        mozilla::EventStates aStateMask);
 
   void ClearSheets();
 

--- a/layout/style/nsIStyleRuleProcessor.h
+++ b/layout/style/nsIStyleRuleProcessor.h
@@ -115,16 +115,25 @@ public:
    * only, and may err on the side of reporting more dependencies than
    * really exist.
    */
   virtual nsRestyleHint HasAttributeDependentStyle(
       AttributeRuleProcessorData* aData,
       mozilla::RestyleHintData& aRestyleHintDataResult) = 0;
 
   /**
+   * Refresh the rule cascade, if relevant.
+   * Currently only nsCSSRuleProcessor can/should.
+   */
+  virtual void RefreshRuleCascade(nsPresContext* aPresContext)
+  {
+    // No-op unless overridden.
+  };
+
+  /**
    * Do any processing that needs to happen as a result of a change in
    * the characteristics of the medium, and return whether this rule
    * processor's rules have changed (e.g., because of media queries).
    */
   virtual bool MediumFeaturesChanged(nsPresContext* aPresContext) = 0;
 
   /**
    * Report the size of this style rule processor to about:memory.  A

--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -2405,16 +2405,35 @@ nsStyleSet::HasAttributeDependentStyle(E
   if (!(data.mHint & eRestyle_Subtree)) {
     // No point keeping the list of selectors around if we are going to
     // restyle the whole subtree unconditionally.
     aRestyleHintDataResult = Move(data.mHintData);
   }
   return data.mHint;
 }
 
+void
+nsStyleSet::RefreshRuleCascade()
+{
+  NS_ASSERTION(mBatching == 0, "rule processors out of date");
+
+  nsPresContext* presContext = PresContext();
+
+  for (nsIStyleRuleProcessor* processor : mRuleProcessors) {
+    if (!processor) {
+      continue;
+    }
+    processor->RefreshRuleCascade(presContext);
+  }
+
+  for (nsIStyleRuleProcessor* processor : mScopedDocSheetRuleProcessors) {
+    processor->RefreshRuleCascade(presContext);
+  }
+}
+
 bool
 nsStyleSet::MediumFeaturesChanged()
 {
   NS_ASSERTION(mBatching == 0, "rule processors out of date");
 
   // We can't use WalkRuleProcessors without a content node.
   nsPresContext* presContext = PresContext();
   bool stylesChanged = false;

--- a/layout/style/nsStyleSet.h
+++ b/layout/style/nsStyleSet.h
@@ -303,16 +303,21 @@ class nsStyleSet final
                                            nsIAtom*       aAttribute,
                                            int32_t        aModType,
                                            bool           aAttrHasChanged,
                                            const nsAttrValue* aOtherValue,
                                            mozilla::RestyleHintData&
                                              aRestyleHintDataResult);
 
   /*
+   * Refresh the rule cascade in all associated rule processors.
+   */
+  void RefreshRuleCascade();
+
+  /*
    * Do any processing that needs to happen as a result of a change in
    * the characteristics of the medium, and return whether style rules
    * may have changed as a result.
    */
   bool MediumFeaturesChanged();
 
   // APIs for registering objects that can supply additional
   // rules during processing.