diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 1b5f36f8e0f..f2a3be1d17a 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -806,10 +806,9 @@ nsPresContext::AppUnitsPerDevPixelChanged()
 
   mDeviceContext->FlushFontCache();
 
-  // All cached style data must be recomputed.
   if (HasCachedStyleData()) {
-    MediaFeatureValuesChanged(true);
-    RebuildAllStyleData(NS_STYLE_HINT_REFLOW);
+    // All cached style data must be recomputed.
+    MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
   }
 }
 
@@ -1572,13 +1571,11 @@ nsPresContext::ThemeChangedInternal()
   nsCSSRuleProcessor::FreeSystemMetrics();
 
   // Changes to system metrics can change media queries on them.
-  MediaFeatureValuesChanged(true);
-
   // Changes in theme can change system colors (whose changes are
   // properly reflected in computed style data), system fonts (whose
   // changes are not), and -moz-appearance (whose changes likewise are
   // not), so we need to reflow.
-  RebuildAllStyleData(NS_STYLE_HINT_REFLOW);
+  MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
 }
 
 void
@@ -1680,17 +1677,22 @@ nsPresContext::PostRebuildAllStyleDataEvent(nsChangeHint aExtraHint)
 }
 
 void
-nsPresContext::MediaFeatureValuesChanged(bool aCallerWillRebuildStyleData)
+nsPresContext::MediaFeatureValuesChanged(StyleRebuildType aShouldRebuild,
+                                         nsChangeHint aChangeHint)
 {
+  NS_ASSERTION(aShouldRebuild == eAlwaysRebuildStyle || aChangeHint == 0,
+               "If you don't know if we need a rebuild, how can you provide a hint?");
+
   mPendingMediaFeatureValuesChanged = false;
 
   // MediumFeaturesChanged updates the applied rules, so it always gets called.
   bool mediaFeaturesDidChange = mShell ? mShell->StyleSet()->MediumFeaturesChanged(this)
                                        : false;
 
-  if (!aCallerWillRebuildStyleData &&
-      (mediaFeaturesDidChange || (mUsesViewportUnits && mPendingViewportChange))) {
-    RebuildAllStyleData(nsChangeHint(0));
+  if (aShouldRebuild == eAlwaysRebuildStyle ||
+      mediaFeaturesDidChange ||
+      (mUsesViewportUnits && mPendingViewportChange)) {
+    RebuildAllStyleData(aChangeHint);
   }
 
   mPendingViewportChange = false;
@@ -1769,7 +1771,7 @@ nsPresContext::HandleMediaFeatureValuesChangedEvent()
   // Null-check mShell in case the shell has been destroyed (and the
   // event is the only thing holding the pres context alive).
   if (mPendingMediaFeatureValuesChanged && mShell) {
-    MediaFeatureValuesChanged(false);
+    MediaFeatureValuesChanged(eRebuildStyleIfNeeded);
   }
 }
 
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 0e28fe73dca..138b35d4f8c 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -157,6 +157,12 @@ public:
     eContext_PageLayout    // paginated & editable.
   };
 
+  // Policies for rebuilding style data.
+  enum StyleRebuildType {
+    eRebuildStyleIfNeeded,
+    eAlwaysRebuildStyle
+  };
+
   nsPresContext(nsIDocument* aDocument, nsPresContextType aType) NS_HIDDEN;
 
   /**
@@ -248,12 +254,13 @@ public:
    */
   void PostRebuildAllStyleDataEvent(nsChangeHint aExtraHint);
 
-  void MediaFeatureValuesChanged(bool aCallerWillRebuildStyleData);
+  void MediaFeatureValuesChanged(StyleRebuildType aShouldRebuild,
+                                 nsChangeHint aChangeHint = nsChangeHint(0));
   void PostMediaFeatureValuesChangedEvent();
   NS_HIDDEN_(void) HandleMediaFeatureValuesChangedEvent();
   void FlushPendingMediaFeatureValuesChanged() {
     if (mPendingMediaFeatureValuesChanged)
-      MediaFeatureValuesChanged(false);
+      MediaFeatureValuesChanged(eRebuildStyleIfNeeded);
   }
 
   /**
@@ -493,8 +500,7 @@ public:
     if (HasCachedStyleData()) {
       // Media queries could have changed, since we changed the meaning
       // of 'em' units in them.
-      MediaFeatureValuesChanged(true);
-      RebuildAllStyleData(NS_STYLE_HINT_REFLOW);
+      MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
     }
   }
 
@@ -515,8 +521,7 @@ public:
     if (HasCachedStyleData()) {
       // Media queries could have changed, since we changed the meaning
       // of 'em' units in them.
-      MediaFeatureValuesChanged(true);
-      RebuildAllStyleData(NS_STYLE_HINT_REFLOW);
+      MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
     }
   }