diff --git a/gfx/thebes/gfxUserFontSet.cpp b/gfx/thebes/gfxUserFontSet.cpp
index 7d1cc222fc0..6fe26c44468 100644
--- a/gfx/thebes/gfxUserFontSet.cpp
+++ b/gfx/thebes/gfxUserFontSet.cpp
@@ -126,6 +126,8 @@ gfxUserFontEntry::gfxUserFontEntry(gfxUserFontSet* aFontSet,
       mLoader(nullptr),
       mFontSet(aFontSet)
 {
+    MOZ_ASSERT(aWeight != 0,
+               "aWeight must not be 0; use NS_FONT_WEIGHT_NORMAL instead");
     mIsUserFontContainer = true;
     mSrcList = aFontFaceSrcList;
     mSrcIndex = 0;
@@ -727,8 +729,6 @@ gfxUserFontSet::CreateFontFace(const nsTArray<gfxFontFaceSrc>& aFontFaceSrcList,
                                uint32_t aLanguageOverride,
                                gfxSparseBitSet* aUnicodeRanges)
 {
-    MOZ_ASSERT(aWeight != 0,
-               "aWeight must not be 0; use NS_FONT_WEIGHT_NORMAL instead");
 
     nsRefPtr<gfxUserFontEntry> userFontEntry =
         new gfxUserFontEntry(this, aFontFaceSrcList, aWeight,
diff --git a/gfx/thebes/gfxUserFontSet.h b/gfx/thebes/gfxUserFontSet.h
index ec64913df55..1eae77c9c5a 100644
--- a/gfx/thebes/gfxUserFontSet.h
+++ b/gfx/thebes/gfxUserFontSet.h
@@ -154,14 +154,14 @@ public:
     // italic style = constants in gfxFontConstants.h, e.g. NS_FONT_STYLE_NORMAL
     // language override = result of calling gfxFontStyle::ParseFontLanguageOverride
     // TODO: support for unicode ranges not yet implemented
-    already_AddRefed<gfxUserFontEntry> CreateFontFace(
+    virtual already_AddRefed<gfxUserFontEntry> CreateFontFace(
                               const nsTArray<gfxFontFaceSrc>& aFontFaceSrcList,
                               uint32_t aWeight,
                               int32_t aStretch,
                               uint32_t aItalicStyle,
                               const nsTArray<gfxFontFeature>& aFeatureSettings,
                               uint32_t aLanguageOverride,
-                              gfxSparseBitSet* aUnicodeRanges);
+                              gfxSparseBitSet* aUnicodeRanges) = 0;
 
     // creates a font face for the specified family, or returns an existing
     // matching entry on the family if there is one
diff --git a/layout/style/FontFace.h b/layout/style/FontFace.h
index f712204a388..685c0e71802 100644
--- a/layout/style/FontFace.h
+++ b/layout/style/FontFace.h
@@ -6,8 +6,9 @@
 #ifndef mozilla_dom_FontFace_h
 #define mozilla_dom_FontFace_h
 
-#include "nsWrapperCache.h"
 #include "mozilla/dom/FontFaceBinding.h"
+#include "gfxUserFontSet.h"
+#include "nsWrapperCache.h"
 
 namespace mozilla {
 namespace dom {
@@ -24,6 +25,21 @@ class FontFace MOZ_FINAL : public nsISupports,
                            public nsWrapperCache
 {
 public:
+  class Entry MOZ_FINAL : public gfxUserFontEntry {
+  public:
+    Entry(gfxUserFontSet* aFontSet,
+          const nsTArray<gfxFontFaceSrc>& aFontFaceSrcList,
+          uint32_t aWeight,
+          int32_t aStretch,
+          uint32_t aItalicStyle,
+          const nsTArray<gfxFontFeature>& aFeatureSettings,
+          uint32_t aLanguageOverride,
+          gfxSparseBitSet* aUnicodeRanges)
+      : gfxUserFontEntry(aFontSet, aFontFaceSrcList, aWeight, aStretch,
+                         aItalicStyle, aFeatureSettings, aLanguageOverride,
+                         aUnicodeRanges) {}
+  };
+
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(FontFace)
 
diff --git a/layout/style/FontFaceSet.cpp b/layout/style/FontFaceSet.cpp
index 1dc61d4b777..e4951ea27d3 100644
--- a/layout/style/FontFaceSet.cpp
+++ b/layout/style/FontFaceSet.cpp
@@ -11,6 +11,7 @@
 #endif /* MOZ_LOGGING */
 #include "prlog.h"
 
+#include "mozilla/dom/FontFace.h"
 #include "mozilla/dom/FontFaceSetBinding.h"
 #include "mozilla/dom/Promise.h"
 #include "nsCrossSiteListenerProxy.h"
@@ -987,3 +988,19 @@ FontFaceSet::UserFontSet::DoRebuildUserFontSet()
   }
   mFontFaceSet->DoRebuildUserFontSet();
 }
+
+/* virtual */ already_AddRefed<gfxUserFontEntry>
+FontFaceSet::UserFontSet::CreateFontFace(
+                               const nsTArray<gfxFontFaceSrc>& aFontFaceSrcList,
+                               uint32_t aWeight,
+                               int32_t aStretch,
+                               uint32_t aItalicStyle,
+                               const nsTArray<gfxFontFeature>& aFeatureSettings,
+                               uint32_t aLanguageOverride,
+                               gfxSparseBitSet* aUnicodeRanges)
+{
+  nsRefPtr<gfxUserFontEntry> entry =
+    new FontFace::Entry(this, aFontFaceSrcList, aWeight, aStretch, aItalicStyle,
+                        aFeatureSettings, aLanguageOverride, aUnicodeRanges);
+  return entry.forget();
+}
diff --git a/layout/style/FontFaceSet.h b/layout/style/FontFaceSet.h
index d9ae52b2806..1eca17918ab 100644
--- a/layout/style/FontFaceSet.h
+++ b/layout/style/FontFaceSet.h
@@ -72,6 +72,14 @@ public:
                                 uint32_t aFlags = nsIScriptError::errorFlag,
                                 nsresult aStatus = NS_OK) MOZ_OVERRIDE;
     virtual void DoRebuildUserFontSet() MOZ_OVERRIDE;
+    virtual already_AddRefed<gfxUserFontEntry> CreateFontFace(
+                                   const nsTArray<gfxFontFaceSrc>& aFontFaceSrcList,
+                                   uint32_t aWeight,
+                                   int32_t aStretch,
+                                   uint32_t aItalicStyle,
+                                   const nsTArray<gfxFontFeature>& aFeatureSettings,
+                                   uint32_t aLanguageOverride,
+                                   gfxSparseBitSet* aUnicodeRanges) MOZ_OVERRIDE;
 
   private:
     nsRefPtr<FontFaceSet> mFontFaceSet;