--- a/gfx/2d/Factory.cpp
+++ b/gfx/2d/Factory.cpp
@@ -60,16 +60,18 @@
 
 #include "Logging.h"
 
 #include "mozilla/CheckedInt.h"
 
 #ifdef MOZ_ENABLE_FREETYPE
 #  include "ft2build.h"
 #  include FT_FREETYPE_H
+#  include FT_OUTLINE_H
+#  include FT_SYNTHESIS_H
 #endif
 #include "MainThreadUtils.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/StaticPrefs_gfx.h"
 
 #if defined(MOZ_LOGGING)
 GFX2D_API mozilla::LogModule* GetGFX2DLog() {
   static mozilla::LazyLogModule sLog("gfx2d");
@@ -190,16 +192,50 @@ FT_Error mozilla_LoadFTGlyph(FT_Face aFa
 
 void mozilla_LockFTLibrary(FT_Library aFTLibrary) {
   mozilla::gfx::Factory::LockFTLibrary(aFTLibrary);
 }
 
 void mozilla_UnlockFTLibrary(FT_Library aFTLibrary) {
   mozilla::gfx::Factory::UnlockFTLibrary(aFTLibrary);
 }
+
+// Custom version of FT_GlyphSlot_Embolden to be less aggressive with outline
+// fonts than the default implementation in FreeType.
+void mozilla_GlyphSlot_Embolden_Less(FT_GlyphSlot slot) {
+  if (!slot) {
+    return;
+  }
+  if (slot->format != FT_GLYPH_FORMAT_OUTLINE) {
+    // For non-outline glyphs, just fall back to FreeType's function.
+    FT_GlyphSlot_Embolden(slot);
+    return;
+  }
+
+  FT_Face face = slot->face;
+
+  // FT_GlyphSlot_Embolden uses a divisor of 24 here; we'll be only half as
+  // bold.
+  FT_Pos strength =
+      FT_MulFix(face->units_per_EM, face->size->metrics.y_scale) / 48;
+  FT_Outline_Embolden(&slot->outline, strength);
+
+  // Adjust metrics to suit the fattened glyph.
+  if (slot->advance.x) {
+    slot->advance.x += strength;
+  }
+  if (slot->advance.y) {
+    slot->advance.y += strength;
+  }
+  slot->metrics.width += strength;
+  slot->metrics.height += strength;
+  slot->metrics.horiAdvance += strength;
+  slot->metrics.vertAdvance += strength;
+  slot->metrics.horiBearingY += strength;
+}
 }
 #endif
 
 namespace mozilla {
 namespace gfx {
 
 #ifdef MOZ_ENABLE_FREETYPE
 FT_Library Factory::mFTLibrary = nullptr;

--- a/gfx/skia/generate_mozbuild.py
+++ b/gfx/skia/generate_mozbuild.py
@@ -67,16 +67,20 @@ elif CONFIG['CPU_ARCH'] == 'aarch64' and
 DEFINES['SKIA_IMPLEMENTATION'] = 1
 
 if CONFIG['MOZ_ENABLE_SKIA_PDF_SFNTLY']:
     DEFINES['SK_PDF_USE_SFNTLY'] = 1
 
 if CONFIG['MOZ_TREE_FREETYPE']:
     DEFINES['SK_CAN_USE_DLOPEN'] = 0
 
+# Reduce strength of synthetic-emboldening used in the freetype backend
+# (see bug 1600470).
+DEFINES['SK_OUTLINE_EMBOLDEN_DIVISOR'] = 48
+
 # Suppress warnings in third-party code.
 CXXFLAGS += [
     '-Wno-deprecated-declarations',
     '-Wno-overloaded-virtual',
     '-Wno-shadow',
     '-Wno-sign-compare',
     '-Wno-unreachable-code',
     '-Wno-unused-function',

--- a/gfx/skia/skia/src/ports/SkFontHost_cairo.cpp
+++ b/gfx/skia/skia/src/ports/SkFontHost_cairo.cpp
@@ -53,39 +53,37 @@ typedef enum FT_LcdFilter_
 #endif
 
 #ifndef SK_FONTHOST_CAIRO_STANDALONE
 #define SK_FONTHOST_CAIRO_STANDALONE 1
 #endif
 
 static bool gFontHintingEnabled = true;
 static FT_Error (*gSetLcdFilter)(FT_Library, FT_LcdFilter) = nullptr;
-static void (*gGlyphSlotEmbolden)(FT_GlyphSlot) = nullptr;
 
 extern "C"
 {
     void mozilla_LockFTLibrary(FT_Library aLibrary);
     void mozilla_UnlockFTLibrary(FT_Library aLibrary);
     void mozilla_AddRefSharedFTFace(void* aContext);
     void mozilla_ReleaseSharedFTFace(void* aContext, void* aOwner);
     void mozilla_ForgetSharedFTFaceLockOwner(void* aContext, void* aOwner);
     int mozilla_LockSharedFTFace(void* aContext, void* aOwner);
     void mozilla_UnlockSharedFTFace(void* aContext);
     FT_Error mozilla_LoadFTGlyph(FT_Face aFace, uint32_t aGlyphIndex, int32_t aFlags);
+    void mozilla_GlyphSlot_Embolden_Less(FT_GlyphSlot slot);
 }
 
 void SkInitCairoFT(bool fontHintingEnabled)
 {
     gFontHintingEnabled = fontHintingEnabled;
 #if SK_CAN_USE_DLOPEN
     gSetLcdFilter = (FT_Error (*)(FT_Library, FT_LcdFilter))dlsym(RTLD_DEFAULT, "FT_Library_SetLcdFilter");
-    gGlyphSlotEmbolden = (void (*)(FT_GlyphSlot))dlsym(RTLD_DEFAULT, "FT_GlyphSlot_Embolden");
 #else
     gSetLcdFilter = &FT_Library_SetLcdFilter;
-    gGlyphSlotEmbolden = &FT_GlyphSlot_Embolden;
 #endif
     // FT_Library_SetLcdFilter may be provided but have no effect if FreeType
     // is built without FT_CONFIG_OPTION_SUBPIXEL_RENDERING.
     if (gSetLcdFilter &&
         gSetLcdFilter(nullptr, FT_LCD_FILTER_NONE) == FT_Err_Unimplemented_Feature) {
         gSetLcdFilter = nullptr;
     }
 }
@@ -481,19 +479,19 @@ unsigned SkScalerContext_CairoFT::genera
 bool SkScalerContext_CairoFT::generateAdvance(SkGlyph* glyph)
 {
     generateMetrics(glyph);
     return !glyph->isEmpty();
 }
 
 void SkScalerContext_CairoFT::prepareGlyph(FT_GlyphSlot glyph)
 {
-    if (fRec.fFlags & SkScalerContext::kEmbolden_Flag &&
-        gGlyphSlotEmbolden) {
-        gGlyphSlotEmbolden(glyph);
+    if (fRec.fFlags & SkScalerContext::kEmbolden_Flag) {
+        // Not FT_GlyphSlot_Embolden because we want a less extreme effect.
+        mozilla_GlyphSlot_Embolden_Less(glyph);
     }
 }
 
 void SkScalerContext_CairoFT::generateMetrics(SkGlyph* glyph)
 {
     glyph->fMaskFormat = fRec.fMaskFormat;
 
     glyph->zeroMetrics();

--- a/gfx/thebes/gfxFT2FontBase.cpp
+++ b/gfx/thebes/gfxFT2FontBase.cpp
@@ -495,16 +495,26 @@ bool gfxFT2FontBase::ShouldRoundXOffset(
                    gfx_text_subpixel_position_force_enabled_AtStartup()));
 }
 
 FT_Vector gfxFT2FontBase::GetEmboldenStrength(FT_Face aFace) {
   FT_Vector strength = {0, 0};
   if (!mEmbolden) {
     return strength;
   }
+
+  // If it's an outline glyph, we'll be using GlyphSlot_Embolden_Less,
+  // so we need to match its strength here.
+  if (aFace->glyph->format == FT_GLYPH_FORMAT_OUTLINE) {
+    strength.x =
+        FT_MulFix(aFace->units_per_EM, aFace->size->metrics.y_scale) / 48;
+    strength.y = strength.x;
+    return strength;
+  }
+
   // This is the embolden "strength" used by FT_GlyphSlot_Embolden.
   strength.x =
       FT_MulFix(aFace->units_per_EM, aFace->size->metrics.y_scale) / 24;
   strength.y = strength.x;
   if (aFace->glyph->format == FT_GLYPH_FORMAT_BITMAP) {
     strength.x &= -64;
     if (!strength.x) {
       strength.x = 64;

--- a/gfx/wr/webrender/src/platform/unix/font.rs
+++ b/gfx/wr/webrender/src/platform/unix/font.rs
@@ -101,17 +101,17 @@ macro_rules! ft_dyn_fn {
     ($($proto:tt)+) => { extern "C" { fn $($proto)+; } }
 }
 
 ft_dyn_fn!(FT_Get_MM_Var(face: FT_Face, desc: *mut *mut FT_MM_Var) -> FT_Error);
 ft_dyn_fn!(FT_Done_MM_Var(library: FT_Library, desc: *mut FT_MM_Var) -> FT_Error);
 ft_dyn_fn!(FT_Set_Var_Design_Coordinates(face: FT_Face, num_vals: FT_UInt, vals: *mut FT_Fixed) -> FT_Error);
 
 extern "C" {
-    fn FT_GlyphSlot_Embolden(slot: FT_GlyphSlot);
+    fn mozilla_GlyphSlot_Embolden_Less(slot: FT_GlyphSlot);
 }
 
 enum FontFile {
     Pathname(CString),
     Data(Arc<Vec<u8>>),
 }
 
 struct FontFace {
@@ -490,17 +490,17 @@ impl FontContext {
             );
             return None;
         }
 
         let slot = unsafe { (*face).glyph };
         assert!(slot != ptr::null_mut());
 
         if font.flags.contains(FontInstanceFlags::SYNTHETIC_BOLD) {
-            unsafe { FT_GlyphSlot_Embolden(slot) };
+            unsafe { mozilla_GlyphSlot_Embolden_Less(slot) };
         }
 
         let format = unsafe { (*slot).format };
         match format {
             FT_Glyph_Format::FT_GLYPH_FORMAT_BITMAP => {
                 let y_size = unsafe { (*(*(*slot).face).size).metrics.y_ppem };
                 Some((slot, req_size as f32 / y_size as f32))
             }

--- a/layout/reftests/css-break/reftest.list
+++ b/layout/reftests/css-break/reftest.list
@@ -1,11 +1,11 @@
 == box-decoration-break-1.html box-decoration-break-1-ref.html
 fuzzy(0-1,0-20) fuzzy-if(skiaContent,0-1,0-700) == box-decoration-break-with-inset-box-shadow-1.html box-decoration-break-with-inset-box-shadow-1-ref.html
-skip-if(verify) fuzzy(0-45,0-460) fuzzy-if(skiaContent,0-57,0-439) fuzzy-if(Android,0-57,0-1330) random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == box-decoration-break-with-outset-box-shadow-1.html box-decoration-break-with-outset-box-shadow-1-ref.html # Bug 1386543, bug 1392106
+skip-if(verify) fuzzy(0-45,0-460) fuzzy-if(skiaContent,0-57,0-439) fuzzy-if(Android,0-70,0-1330) random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == box-decoration-break-with-outset-box-shadow-1.html box-decoration-break-with-outset-box-shadow-1-ref.html # Bug 1386543, bug 1392106
 random-if(!gtkWidget||webrender) == box-decoration-break-border-image.html box-decoration-break-border-image-ref.html
 == box-decoration-break-block-border-padding.html box-decoration-break-block-border-padding-ref.html
 == box-decoration-break-block-margin.html box-decoration-break-block-margin-ref.html
 fuzzy-if(!Android,0-1,0-62) fuzzy-if(Android,0-8,0-6627) random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == box-decoration-break-first-letter.html box-decoration-break-first-letter-ref.html #Bug 1313773 # Bug 1392106
 == box-decoration-break-with-bidi.html box-decoration-break-with-bidi-ref.html
 random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == box-decoration-break-bug-1235152.html box-decoration-break-bug-1235152-ref.html # Bug 1392106
 random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == box-decoration-break-bug-1249913.html box-decoration-break-bug-1249913-ref.html # Bug 1392106
 == vertical-wm-001.html vertical-wm-001-ref.html

--- a/layout/reftests/text/synthetic-bold-metrics-01-notref.html
+++ b/layout/reftests/text/synthetic-bold-metrics-01-notref.html
@@ -3,17 +3,17 @@
 <head>
 <style type="text/css">
 @font-face {
   font-family: dejavu;
   src: url(../fonts/dejavu-sans/DejaVuSans.ttf);
 }
 
 p {
-  font-family: dejavu; /* family with only a single weight */
+  font: 32px dejavu; /* family with only a single weight */
 }
 
 .test {
   color: white; /* hide the text, we're only comparing metrics */
 }
 </style>
 </head>
 <body>

--- a/layout/reftests/text/synthetic-bold-metrics-01.html
+++ b/layout/reftests/text/synthetic-bold-metrics-01.html
@@ -3,17 +3,17 @@
 <head>
 <style type="text/css">
 @font-face {
   font-family: dejavu;
   src: url(../fonts/dejavu-sans/DejaVuSans.ttf);
 }
 
 p {
-  font-family: dejavu; /* family with only a single weight */
+  font: 32px dejavu; /* family with only a single weight */
 }
 
 .test {
   color: white; /* hide the text, we're only comparing metrics */
   font-weight: bold; /* synthetic bold will be used */
 }
 </style>
 </head>