--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -4377,17 +4377,17 @@ nsDocShell::GetWindow()
 NS_IMETHODIMP
 nsDocShell::SetDeviceSizeIsPageSize(bool aValue)
 {
     if (mDeviceSizeIsPageSize != aValue) {
       mDeviceSizeIsPageSize = aValue;
       nsRefPtr<nsPresContext> presContext;
       GetPresContext(getter_AddRefs(presContext));
       if (presContext) {
-          presContext->MediaFeatureValuesChanged(presContext->eAlwaysRebuildStyle);
+          presContext->MediaFeatureValuesChanged(eRestyle_Subtree);
       }
     }
     return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDocShell::GetDeviceSizeIsPageSize(bool* aValue)
 {

--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -842,17 +842,17 @@ nsPresContext::AppUnitsPerDevPixelChange
   InvalidatePaintedLayers();
 
   if (mDeviceContext) {
     mDeviceContext->FlushFontCache();
   }
 
   if (HasCachedStyleData()) {
     // All cached style data must be recomputed.
-    MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
+    MediaFeatureValuesChanged(eRestyle_Subtree, NS_STYLE_HINT_REFLOW);
   }
 
   mCurAppUnitsPerDevPixel = AppUnitsPerDevPixel();
 }
 
 void
 nsPresContext::PreferenceChanged(const char* aPrefName)
 {
@@ -1706,17 +1706,17 @@ nsPresContext::ThemeChangedInternal()
   // This will force the system metrics to be generated the next time they're used
   nsCSSRuleProcessor::FreeSystemMetrics();
 
   // Changes to system metrics can change media queries on them.
   // 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.
-  MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
+  MediaFeatureValuesChanged(eRestyle_Subtree, NS_STYLE_HINT_REFLOW);
 }
 
 void
 nsPresContext::SysColorChanged()
 {
   if (!mPendingSysColorChanged) {
     sLookAndFeelChanged = true;
     nsCOMPtr<nsIRunnable> ev =
@@ -1835,26 +1835,26 @@ nsPresContext::EmulateMedium(const nsASt
   nsIAtom* previousMedium = Medium();
   mIsEmulatingMedia = true;
 
   nsAutoString mediaType;
   nsContentUtils::ASCIIToLower(aMediaType, mediaType);
 
   mMediaEmulated = do_GetAtom(mediaType);
   if (mMediaEmulated != previousMedium && mShell) {
-    MediaFeatureValuesChanged(eRebuildStyleIfNeeded, nsChangeHint(0));
+    MediaFeatureValuesChanged(nsRestyleHint(0), nsChangeHint(0));
   }
 }
 
 void nsPresContext::StopEmulatingMedium()
 {
   nsIAtom* previousMedium = Medium();
   mIsEmulatingMedia = false;
   if (Medium() != previousMedium) {
-    MediaFeatureValuesChanged(eRebuildStyleIfNeeded, nsChangeHint(0));
+    MediaFeatureValuesChanged(nsRestyleHint(0), nsChangeHint(0));
   }
 }
 
 void
 nsPresContext::RebuildAllStyleData(nsChangeHint aExtraHint,
                                    nsRestyleHint aRestyleHint)
 {
   if (!mShell) {
@@ -1877,28 +1877,21 @@ nsPresContext::PostRebuildAllStyleDataEv
   if (!mShell) {
     // We must have been torn down. Nothing to do here.
     return;
   }
   RestyleManager()->PostRebuildAllStyleDataEvent(aExtraHint, aRestyleHint);
 }
 
 void
-nsPresContext::MediaFeatureValuesChanged(StyleRebuildType aShouldRebuild,
+nsPresContext::MediaFeatureValuesChanged(nsRestyleHint aRestyleHint,
                                          nsChangeHint aChangeHint)
 {
   mPendingMediaFeatureValuesChanged = false;
 
-  nsRestyleHint aRestyleHint = nsRestyleHint(0);
-
-  if (aShouldRebuild == eAlwaysRebuildStyle) {
-    // FIXME: Pass restyle hint from caller.
-    aRestyleHint |= eRestyle_Subtree;
-  }
-
   // MediumFeaturesChanged updates the applied rules, so it always gets called.
   if (mShell && mShell->StyleSet()->MediumFeaturesChanged(this)) {
     aRestyleHint |= eRestyle_Subtree;
   }
 
   if (mUsesViewportUnits && mPendingViewportChange) {
     // Rebuild all style data without rerunning selector matching.
     aRestyleHint |= eRestyle_ForceDescendants;
@@ -1974,17 +1967,17 @@ nsPresContext::PostMediaFeatureValuesCha
 }
 
 void
 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(eRebuildStyleIfNeeded);
+    MediaFeatureValuesChanged(nsRestyleHint(0));
   }
 }
 
 already_AddRefed<MediaQueryList>
 nsPresContext::MatchMedia(const nsAString& aMediaQueryList)
 {
   nsRefPtr<MediaQueryList> result = new MediaQueryList(this, aMediaQueryList);
 

--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -146,22 +146,16 @@ public:
 
   enum nsPresContextType {
     eContext_Galley,       // unpaginated screen presentation
     eContext_PrintPreview, // paginated screen presentation
     eContext_Print,        // paginated printer presentation
     eContext_PageLayout    // paginated & editable.
   };
 
-  // Policies for rebuilding style data.
-  enum StyleRebuildType {
-    eRebuildStyleIfNeeded,
-    eAlwaysRebuildStyle
-  };
-
   nsPresContext(nsIDocument* aDocument, nsPresContextType aType);
 
   /**
    * Initialize the presentation context from a particular device.
    */
   nsresult Init(nsDeviceContext* aDeviceContext);
 
   /**
@@ -260,23 +254,43 @@ public:
   void RebuildAllStyleData(nsChangeHint aExtraHint, nsRestyleHint aRestyleHint);
   /**
    * Just like RebuildAllStyleData, except (1) asynchronous and (2) it
    * doesn't rebuild the user font set.
    */
   void PostRebuildAllStyleDataEvent(nsChangeHint aExtraHint,
                                     nsRestyleHint aRestyleHint);
 
-  void MediaFeatureValuesChanged(StyleRebuildType aShouldRebuild,
+  /**
+   * 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
+   *    changed).  (RebuildAllStyleData always adds
+   *    eRestyle_ForceDescendants internally, so here we're only using
+   *    it to distinguish from nsRestyleHint(0) whether we need to call
+   *    RebuildAllStyleData at all.)
+   *  * eRestyle_Subtree to force rebuilding of style data with
+   *    rerunning of selector matching
+   *
+   * For aChangeHint, see RestyleManager::RebuildAllStyleData.  (Passing
+   * a nonzero aChangeHint forces rebuilding style data even if
+   * nsRestyleHint(0) is passed.)
+   */
+  void MediaFeatureValuesChanged(nsRestyleHint aRestyleHint,
                                  nsChangeHint aChangeHint = nsChangeHint(0));
   void PostMediaFeatureValuesChangedEvent();
   void HandleMediaFeatureValuesChangedEvent();
   void FlushPendingMediaFeatureValuesChanged() {
     if (mPendingMediaFeatureValuesChanged)
-      MediaFeatureValuesChanged(eRebuildStyleIfNeeded);
+      MediaFeatureValuesChanged(nsRestyleHint(0));
   }
 
   /**
    * Support for window.matchMedia()
    */
   already_AddRefed<mozilla::dom::MediaQueryList>
     MatchMedia(const nsAString& aMediaQueryList);
 
@@ -530,17 +544,17 @@ public:
   void SetTextZoom(float aZoom) {
     if (aZoom == mTextZoom)
       return;
 
     mTextZoom = aZoom;
     if (HasCachedStyleData()) {
       // Media queries could have changed, since we changed the meaning
       // of 'em' units in them.
-      MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
+      MediaFeatureValuesChanged(eRestyle_Subtree, NS_STYLE_HINT_REFLOW);
     }
   }
 
   /**
    * Get the minimum font size for the specified language. If aLanguage
    * is nullptr, then the document's language is used.  This combines
    * the language-specific global preference with the per-presentation
    * base minimum font size.
@@ -565,17 +579,17 @@ public:
   void SetBaseMinFontSize(int32_t aMinFontSize) {
     if (aMinFontSize == mBaseMinFontSize)
       return;
 
     mBaseMinFontSize = aMinFontSize;
     if (HasCachedStyleData()) {
       // Media queries could have changed, since we changed the meaning
       // of 'em' units in them.
-      MediaFeatureValuesChanged(eAlwaysRebuildStyle, NS_STYLE_HINT_REFLOW);
+      MediaFeatureValuesChanged(eRestyle_Subtree, NS_STYLE_HINT_REFLOW);
     }
   }
 
   float GetFullZoom() { return mFullZoom; }
   void SetFullZoom(float aZoom);
 
   nscoord GetAutoQualityMinFontSize() {
     return DevPixelsToAppUnits(mAutoQualityMinFontSizePixelsPref);