diff --git a/layout/build/nsLayoutStatics.cpp b/layout/build/nsLayoutStatics.cpp
index 0a082ed82289..a0686e6ca9c4 100644
--- a/layout/build/nsLayoutStatics.cpp
+++ b/layout/build/nsLayoutStatics.cpp
@@ -132,7 +132,6 @@ using namespace mozilla::system;
 #include "mozilla/ServoBindings.h"
 #include "mozilla/StaticPresData.h"
 #include "mozilla/dom/WebIDLGlobalNameHash.h"
-#include "mozilla/URLExtraData.h"
 
 using namespace mozilla;
 using namespace mozilla::net;
@@ -313,8 +312,7 @@ nsLayoutStatics::Initialize()
   mozilla::dom::WebCryptoThreadPool::Initialize();
 
 #ifdef MOZ_STYLO
-  URLExtraData::InitDummy();
-  Servo_Initialize(URLExtraData::Dummy());
+  InitializeServo();
 #endif
 
 #ifndef MOZ_WIDGET_ANDROID
@@ -332,7 +330,7 @@ nsLayoutStatics::Shutdown()
   // memory reporter manager.
 
 #ifdef MOZ_STYLO
-  Servo_Shutdown();
+  ShutdownServo();
   URLExtraData::ReleaseDummy();
 #endif
 
diff --git a/layout/style/ServoBindings.cpp b/layout/style/ServoBindings.cpp
index f265b4691ff8..a20d220e843a 100644
--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -27,6 +27,7 @@
 #include "nsIPresShell.h"
 #include "nsIPresShellInlines.h"
 #include "nsIPrincipal.h"
+#include "nsFontMetrics.h"
 #include "nsMappedAttributes.h"
 #include "nsMediaFeatures.h"
 #include "nsNameSpaceManager.h"
@@ -41,6 +42,7 @@
 #include "mozilla/EffectSet.h"
 #include "mozilla/EventStates.h"
 #include "mozilla/Keyframe.h"
+#include "mozilla/Mutex.h"
 #include "mozilla/ServoElementSnapshot.h"
 #include "mozilla/ServoRestyleManager.h"
 #include "mozilla/StyleAnimationValue.h"
@@ -51,6 +53,7 @@
 #include "mozilla/dom/HTMLTableCellElement.h"
 #include "mozilla/dom/HTMLBodyElement.h"
 #include "mozilla/LookAndFeel.h"
+#include "mozilla/URLExtraData.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -1609,6 +1612,48 @@ Gecko_GetBaseSize(nsIAtom* aLanguage)
   return sizes;
 }
 
+static Mutex* sServoFontMetricsLock = nullptr;
+
+void
+InitializeServo()
+{
+  URLExtraData::InitDummy();
+  Servo_Initialize(URLExtraData::Dummy());
+
+  sServoFontMetricsLock = new Mutex("Gecko_GetFontMetrics");
+}
+
+void
+ShutdownServo()
+{
+  delete sServoFontMetricsLock;
+  Servo_Shutdown();
+}
+
+GeckoFontMetrics
+Gecko_GetFontMetrics(RawGeckoPresContextBorrowed aPresContext,
+                     bool aIsVertical,
+                     const nsStyleFont* aFont,
+                     nscoord aFontSize,
+                     bool aUseUserFontSet)
+{
+  MutexAutoLock lock(*sServoFontMetricsLock);
+  aPresContext->SetUsesExChUnits(true);
+  GeckoFontMetrics ret;
+  // Safe because we are locked, and this function is only
+  // ever called from Servo parallel traversal
+  MOZ_ASSERT(ServoStyleSet::IsInServoTraversal());
+  nsPresContext* presContext = const_cast<nsPresContext*>(aPresContext);
+  RefPtr<nsFontMetrics> fm = nsRuleNode::GetMetricsFor(presContext, aIsVertical,
+                                                       aFont, aFontSize,
+                                                       aUseUserFontSet);
+  ret.mXSize = fm->XHeight();
+  gfxFloat zeroWidth = fm->GetThebesFontGroup()->GetFirstValidFont()->
+                           GetMetrics(fm->Orientation()).zeroOrAveCharWidth;
+  ret.mChSize = ceil(aPresContext->AppUnitsPerDevPixel() * zeroWidth);
+  return ret;
+}
+
 void
 Gecko_LoadStyleSheet(css::Loader* aLoader,
                      ServoStyleSheet* aParent,
diff --git a/layout/style/ServoBindings.h b/layout/style/ServoBindings.h
index 66809c19af9c..d5b8becf66f2 100644
--- a/layout/style/ServoBindings.h
+++ b/layout/style/ServoBindings.h
@@ -399,6 +399,20 @@ void Gecko_nsStyleFont_SetLang(nsStyleFont* font, nsIAtom* atom);
 void Gecko_nsStyleFont_CopyLangFrom(nsStyleFont* aFont, const nsStyleFont* aSource);
 FontSizePrefs Gecko_GetBaseSize(nsIAtom* lang);
 
+struct GeckoFontMetrics
+{
+  nscoord mChSize;
+  nscoord mXSize;
+};
+
+GeckoFontMetrics Gecko_GetFontMetrics(RawGeckoPresContextBorrowed pres_context,
+                                      bool is_vertical,
+                                      const nsStyleFont* font,
+                                      nscoord font_size,
+                                      bool use_user_font_set);
+void InitializeServo();
+void ShutdownServo();
+
 const nsMediaFeature* Gecko_GetMediaFeatures();
 
 // Font face rule
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index 929a2d84e84f..bef63b9e4adc 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -614,16 +614,16 @@ static nscoord CalcLengthWith(const nsCSSValue& aValue,
     case eCSSUnit_XHeight: {
       aPresContext->SetUsesExChUnits(true);
       RefPtr<nsFontMetrics> fm =
-        GetMetricsFor(aPresContext, aStyleContext, styleFont,
-                      aFontSize, aUseUserFontSet);
+        nsRuleNode::GetMetricsFor(aPresContext, aStyleContext, styleFont,
+                                  aFontSize, aUseUserFontSet);
       aConditions.SetUncacheable();
       return ScaleCoordRound(aValue, float(fm->XHeight()));
     }
     case eCSSUnit_Char: {
       aPresContext->SetUsesExChUnits(true);
       RefPtr<nsFontMetrics> fm =
-        GetMetricsFor(aPresContext, aStyleContext, styleFont,
-                      aFontSize, aUseUserFontSet);
+        nsRuleNode::GetMetricsFor(aPresContext, aStyleContext, styleFont,
+                                  aFontSize, aUseUserFontSet);
       gfxFloat zeroWidth =
         fm->GetThebesFontGroup()->GetFirstValidFont()->
           GetMetrics(fm->Orientation()).zeroOrAveCharWidth;