--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -269,17 +269,16 @@ nsPresContext::nsPresContext(nsIDocument
     mPendingThemeChanged(false),
     mPendingUIResolutionChanged(false),
     mPendingMediaFeatureValuesChanged(false),
     mPrefChangePendingNeedsReflow(false),
     mIsEmulatingMedia(false),
     mIsGlyph(false),
     mUsesRootEMUnits(false),
     mUsesExChUnits(false),
-    mUsesViewportUnits(false),
     mPendingViewportChange(false),
     mCounterStylesDirty(true),
     mPostedFlushCounterStyles(false),
     mSuppressResizeReflow(false),
     mIsVisual(false),
     mFireAfterPaintEvents(false),
     mIsChrome(false),
     mIsChromeOriginImage(false),
@@ -2054,17 +2053,19 @@ nsPresContext::RebuildAllStyleData(nsCha
 {
   if (!mShell) {
     // We must have been torn down. Nothing to do here.
     return;
   }
 
   mUsesRootEMUnits = false;
   mUsesExChUnits = false;
-  mUsesViewportUnits = false;
+  if (nsStyleSet* styleSet = mShell->StyleSet()->GetAsGecko()) {
+    styleSet->SetUsesViewportUnits(false);
+  }
   mDocument->RebuildUserFontSet();
   RebuildCounterStyles();
 
   RestyleManager()->RebuildAllStyleData(aExtraHint, aRestyleHint);
 }
 
 void
 nsPresContext::PostRebuildAllStyleDataEvent(nsChangeHint aExtraHint,
@@ -2117,28 +2118,16 @@ nsPresContext::MediaFeatureValuesChanged
   mPendingMediaFeatureValuesChanged = false;
 
   // MediumFeaturesChanged updates the applied rules, so it always gets called.
   if (mShell) {
     aRestyleHint |= mShell->
       StyleSet()->MediumFeaturesChanged(mPendingViewportChange);
   }
 
-  if (mPendingViewportChange &&
-      (mUsesViewportUnits || mDocument->IsStyledByServo())) {
-    // Rebuild all style data without rerunning selector matching.
-    //
-    // FIXME(emilio, bug 1328652): We don't set mUsesViewportUnits in stylo yet,
-    // so assume the worst.
-    //
-    // Also, in this case we don't need to do a rebuild of the style data, only
-    // post a restyle.
-    aRestyleHint |= eRestyle_ForceDescendants;
-  }
-
   if (aRestyleHint || aChangeHint) {
     RebuildAllStyleData(aChangeHint, aRestyleHint);
   }
 
   mPendingViewportChange = false;
 
   if (mDocument->IsBeingUsedAsImage()) {
     MOZ_ASSERT(mDocument->MediaQueryLists().isEmpty());

--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -1148,24 +1148,16 @@ public:
   bool UsesExChUnits() const {
     return mUsesExChUnits;
   }
 
   void SetUsesExChUnits(bool aValue) {
     mUsesExChUnits = aValue;
   }
 
-  bool UsesViewportUnits() const {
-    return mUsesViewportUnits;
-  }
-
-  void SetUsesViewportUnits(bool aValue) {
-    mUsesViewportUnits = aValue;
-  }
-
   // true if there are OMTA transition updates for the current document which
   // have been throttled, and therefore some style information may not be up
   // to date
   bool ExistThrottledUpdates() const {
     return mExistThrottledUpdates;
   }
 
   void SetExistThrottledUpdates(bool aExistThrottledUpdates) {
@@ -1462,18 +1454,16 @@ protected:
 
   // Are we currently drawing an SVG glyph?
   unsigned              mIsGlyph : 1;
 
   // Does the associated document use root-em (rem) units?
   unsigned              mUsesRootEMUnits : 1;
   // Does the associated document use ex or ch units?
   unsigned              mUsesExChUnits : 1;
-  // Does the associated document use viewport units (vw/vh/vmin/vmax)?
-  unsigned              mUsesViewportUnits : 1;
 
   // Has there been a change to the viewport's dimensions?
   unsigned              mPendingViewportChange : 1;
 
   // Is the current mCounterStyleManager valid?
   unsigned              mCounterStylesDirty : 1;
   // Do we currently have an event posted to call FlushCounterStyles?
   unsigned              mPostedFlushCounterStyles: 1;

--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -114,16 +114,23 @@ ServoStyleSet::InvalidateStyleForCSSRule
 }
 
 nsRestyleHint
 ServoStyleSet::MediumFeaturesChanged(bool aViewportChanged) const
 {
   if (Servo_StyleSet_MediumFeaturesChanged(mRawSet.get())) {
     return eRestyle_Subtree;
   }
+  if (aViewportChanged) {
+    // Rebuild all style data without rerunning selector matching.
+    //
+    // FIXME(emilio, bug 1328652): We don't set mUsesViewportUnits in stylo yet,
+    // so assume the worst.
+    return eRestyle_ForceDescendants;
+  }
   return nsRestyleHint(0);
 }
 
 size_t
 ServoStyleSet::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   size_t n = aMallocSizeOf(this);
 

--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -466,20 +466,24 @@ nsRuleNode::ApplyMinFontSize(nsStyleFont
       fontSize = aMinFontSize;
     }
   }
   aFont->mFont.size = fontSize;
 }
 
 static nsSize CalcViewportUnitsScale(nsPresContext* aPresContext)
 {
-  // The caller is making use of viewport units, so notify the pres context
+  // The caller is making use of viewport units, so notify the style set
   // that it will need to rebuild the rule tree if the size of the viewport
   // changes.
-  aPresContext->SetUsesViewportUnits(true);
+  // It is possible for this to be called on a Servo-styled document,from
+  // media query evaluation outside stylesheets.
+  if (nsStyleSet* styleSet = aPresContext->StyleSet()->GetAsGecko()) {
+    styleSet->SetUsesViewportUnits(true);
+  }
 
   // The default (when we have 'overflow: auto' on the root element, or
   // trivially for 'overflow: hidden' since we never have scrollbars in that
   // case) is to define the scale of the viewport units without considering
   // scrollbars.
   nsSize viewportSize(aPresContext->GetVisibleArea().Size());
 
   // Check for 'overflow: scroll' styles on the root scroll frame. If we find

--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -214,16 +214,17 @@ nsStyleSet::nsStyleSet()
     mBatching(0),
     mStylesHaveChanged(0),
     mInShutdown(false),
     mInGC(false),
     mAuthorStyleDisabled(false),
     mInReconstruct(false),
     mInitFontFeatureValuesLookup(true),
     mNeedsRestyleAfterEnsureUniqueInner(false),
+    mUsesViewportUnits(false),
     mDirty(0),
     mRootStyleContextCount(0),
 #ifdef DEBUG
     mOldRootNode(nullptr),
 #endif
     mUnusedRuleNodeCount(0)
 {
 }
@@ -2698,16 +2699,20 @@ nsStyleSet::MediumFeaturesChanged(bool a
     bool thisChanged = false;
     mBindingManager->MediumFeaturesChanged(presContext, &thisChanged);
     stylesChanged = stylesChanged || thisChanged;
   }
 
   if (stylesChanged) {
     return eRestyle_Subtree;
   }
+  if (aViewportChanged && mUsesViewportUnits) {
+    // Rebuild all style data without rerunning selector matching.
+    return eRestyle_ForceDescendants;
+  }
   return nsRestyleHint(0);
 }
 
 bool
 nsStyleSet::EnsureUniqueInnerOnCSSSheets()
 {
   AutoTArray<StyleSheet*, 32> queue;
   for (SheetType type : gCSSSheetTypes) {

--- a/layout/style/nsStyleSet.h
+++ b/layout/style/nsStyleSet.h
@@ -486,16 +486,20 @@ class nsStyleSet final
   // to drop any nsCSSSelector pointers it has.
   void ClearSelectors();
 
   // Returns whether aSheetType represents a level of the cascade that uses
   // CSSStyleSheets.  See gCSSSheetTypes in nsStyleSet.cpp for the list
   // of CSS sheet types.
   static bool IsCSSSheetType(mozilla::SheetType aSheetType);
 
+  void SetUsesViewportUnits(bool aValue) {
+    mUsesViewportUnits = aValue;
+  }
+
 private:
   nsStyleSet(const nsStyleSet& aCopy) = delete;
   nsStyleSet& operator=(const nsStyleSet& aCopy) = delete;
 
   // Free all the rules with reference-count zero. This continues iterating
   // over the free list until it is empty, which allows immediate collection
   // of nodes whose reference-count drops to zero during the destruction of
   // a child node. This allows the collection of entire trees at once, since
@@ -628,16 +632,18 @@ private:
   // in mStylesHaveChanged.
   unsigned mStylesHaveChanged : 1;
   unsigned mInShutdown : 1;
   unsigned mInGC : 1;
   unsigned mAuthorStyleDisabled: 1;
   unsigned mInReconstruct : 1;
   unsigned mInitFontFeatureValuesLookup : 1;
   unsigned mNeedsRestyleAfterEnsureUniqueInner : 1;
+  // Does the associated document use viewport units (vw/vh/vmin/vmax)?
+  unsigned mUsesViewportUnits : 1;
   unsigned mDirty : int(mozilla::SheetType::Count);  // one bit per sheet type
 
   uint32_t mRootStyleContextCount;
 
 #ifdef DEBUG
   // In debug builds, we stash a weak pointer here to the old root during
   // reconstruction. During GC, we check for this pointer, and null it out
   // when we encounter it. This allows us to assert that the old root (and