diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 31e30063deee..3ba0aa4b5e10 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -274,7 +274,6 @@ nsPresContext::nsPresContext(nsIDocument* aDocument, nsPresContextType aType)
     mIsGlyph(false),
     mUsesRootEMUnits(false),
     mUsesExChUnits(false),
-    mUsesViewportUnits(false),
     mPendingViewportChange(false),
     mCounterStylesDirty(true),
     mPostedFlushCounterStyles(false),
@@ -2059,7 +2058,9 @@ nsPresContext::RebuildAllStyleData(nsChangeHint aExtraHint,
 
   mUsesRootEMUnits = false;
   mUsesExChUnits = false;
-  mUsesViewportUnits = false;
+  if (nsStyleSet* styleSet = mShell->StyleSet()->GetAsGecko()) {
+    styleSet->SetUsesViewportUnits(false);
+  }
   mDocument->RebuildUserFontSet();
   RebuildCounterStyles();
 
@@ -2122,18 +2123,6 @@ nsPresContext::MediaFeatureValuesChanged(nsRestyleHint aRestyleHint,
       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);
   }
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 8adec1ef4316..158e211ea902 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -1153,14 +1153,6 @@ public:
     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
@@ -1467,8 +1459,6 @@ protected:
   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;
diff --git a/layout/style/ServoStyleSet.cpp b/layout/style/ServoStyleSet.cpp
index 3142d061af89..dac65daad0e3 100644
--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -119,6 +119,13 @@ 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);
 }
 
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index d7ecd0745fc7..97ebc267d1da 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -471,10 +471,14 @@ nsRuleNode::ApplyMinFontSize(nsStyleFont* aFont,
 
 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
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index 6892fd30213a..61431bc65cae 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -219,6 +219,7 @@ nsStyleSet::nsStyleSet()
     mInReconstruct(false),
     mInitFontFeatureValuesLookup(true),
     mNeedsRestyleAfterEnsureUniqueInner(false),
+    mUsesViewportUnits(false),
     mDirty(0),
     mRootStyleContextCount(0),
 #ifdef DEBUG
@@ -2703,6 +2704,10 @@ nsStyleSet::MediumFeaturesChanged(bool aViewportChanged)
   if (stylesChanged) {
     return eRestyle_Subtree;
   }
+  if (aViewportChanged && mUsesViewportUnits) {
+    // Rebuild all style data without rerunning selector matching.
+    return eRestyle_ForceDescendants;
+  }
   return nsRestyleHint(0);
 }
 
diff --git a/layout/style/nsStyleSet.h b/layout/style/nsStyleSet.h
index 6dbf788a4bae..de31877754fc 100644
--- a/layout/style/nsStyleSet.h
+++ b/layout/style/nsStyleSet.h
@@ -491,6 +491,10 @@ class nsStyleSet final
   // 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;
@@ -633,6 +637,8 @@ private:
   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;