diff --git a/dom/smil/nsSMILCSSValueType.cpp b/dom/smil/nsSMILCSSValueType.cpp
index ed89e7710ae4..85fe72085eac 100644
--- a/dom/smil/nsSMILCSSValueType.cpp
+++ b/dom/smil/nsSMILCSSValueType.cpp
@@ -381,7 +381,7 @@ ValueFromStringHelper(nsCSSPropertyID aPropID,
     MOZ_ASSERT(aStyleAnimValue.GetUnit() == StyleAnimationValue::eUnit_Coord,
                "'font-size' value with unexpected style unit");
     aStyleAnimValue.SetCoordValue(aStyleAnimValue.GetCoordValue() /
-                                  aPresContext->TextZoom());
+                                  aPresContext->EffectiveTextZoom());
   }
   return true;
 }
diff --git a/dom/svg/SVGContentUtils.cpp b/dom/svg/SVGContentUtils.cpp
index 5f1ae1ac024b..4ae38ce26441 100644
--- a/dom/svg/SVGContentUtils.cpp
+++ b/dom/svg/SVGContentUtils.cpp
@@ -304,8 +304,8 @@ SVGContentUtils::GetFontSize(nsStyleContext *aStyleContext)
   MOZ_ASSERT(presContext, "NULL pres context in GetFontSize");
 
   nscoord fontSize = aStyleContext->StyleFont()->mSize;
-  return nsPresContext::AppUnitsToFloatCSSPixels(fontSize) / 
-         presContext->TextZoom();
+  return nsPresContext::AppUnitsToFloatCSSPixels(fontSize) /
+         presContext->EffectiveTextZoom();
 }
 
 float
@@ -352,7 +352,7 @@ SVGContentUtils::GetFontXHeight(nsStyleContext *aStyleContext)
 
   nscoord xHeight = fontMetrics->XHeight();
   return nsPresContext::AppUnitsToFloatCSSPixels(xHeight) /
-         presContext->TextZoom();
+         presContext->EffectiveTextZoom();
 }
 nsresult
 SVGContentUtils::ReportToConsole(nsIDocument* doc,
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index 5fb699a60c8e..f283e91c3503 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -807,8 +807,8 @@ nsIPresShell::nsIPresShell()
     , mFontSizeInflationForceEnabled(false)
     , mFontSizeInflationDisabledInMasterProcess(false)
     , mFontSizeInflationEnabled(false)
-    , mPaintingIsFrozen(false)
     , mFontSizeInflationEnabledIsDirty(false)
+    , mPaintingIsFrozen(false)
     , mIsNeverPainting(false)
     , mInFlush(false)
   {}
@@ -11032,12 +11032,18 @@ void
 nsIPresShell::RecomputeFontSizeInflationEnabled()
 {
   mFontSizeInflationEnabledIsDirty = false;
+  mFontSizeInflationEnabled = DetermineFontSizeInflationState();
 
+  HandleSystemFontScale();
+}
+
+bool
+nsIPresShell::DetermineFontSizeInflationState()
+{
   MOZ_ASSERT(mPresContext, "our pres context should not be null");
   if ((FontSizeInflationEmPerLine() == 0 &&
       FontSizeInflationMinTwips() == 0) || mPresContext->IsChrome()) {
-    mFontSizeInflationEnabled = false;
-    return;
+    return false;
   }
 
   // Force-enabling font inflation always trumps the heuristics here.
@@ -11046,15 +11052,13 @@ nsIPresShell::RecomputeFontSizeInflationEnabled()
       // We're in a child process.  Cancel inflation if we're not
       // async-pan zoomed.
       if (!tab->AsyncPanZoomEnabled()) {
-        mFontSizeInflationEnabled = false;
-        return;
+        return false;
       }
     } else if (XRE_IsParentProcess()) {
       // We're in the master process.  Cancel inflation if it's been
       // explicitly disabled.
       if (FontSizeInflationDisabledInMasterProcess()) {
-        mFontSizeInflationEnabled = false;
-        return;
+        return false;
       }
     }
   }
@@ -11079,8 +11083,7 @@ nsIPresShell::RecomputeFontSizeInflationEnabled()
   nsCOMPtr<nsIScreenManager> screenMgr =
     do_GetService("@mozilla.org/gfx/screenmanager;1", &rv);
   if (!NS_SUCCEEDED(rv)) {
-    mFontSizeInflationEnabled = false;
-    return;
+    return false;
   }
 
   nsCOMPtr<nsIScreen> screen;
@@ -11093,12 +11096,11 @@ nsIPresShell::RecomputeFontSizeInflationEnabled()
       GetDocument()->GetViewportInfo(ScreenIntSize(screenWidth, screenHeight));
 
     if (vInf.GetDefaultZoom() >= CSSToScreenScale(1.0f) || vInf.IsAutoSizeEnabled()) {
-      mFontSizeInflationEnabled = false;
-      return;
+      return false;
     }
   }
 
-  mFontSizeInflationEnabled = true;
+  return true;
 }
 
 bool
@@ -11111,6 +11113,23 @@ nsIPresShell::FontSizeInflationEnabled()
   return mFontSizeInflationEnabled;
 }
 
+void
+nsIPresShell::HandleSystemFontScale()
+{
+  float fontScale = nsLayoutUtils::SystemFontScale();
+  if (fontScale == 0.0f) {
+    return;
+  }
+
+  MOZ_ASSERT(mDocument && mPresContext, "our document and pres context should not be null");
+
+  if (!mFontSizeInflationEnabled && !mDocument->IsSyntheticDocument()) {
+    mPresContext->SetSystemFontScale(fontScale);
+  } else {
+    mPresContext->SetSystemFontScale(1.0f);
+  }
+}
+
 void
 PresShell::PausePainting()
 {
diff --git a/layout/base/nsIPresShell.h b/layout/base/nsIPresShell.h
index daaad818e3de..777713f7d4ba 100644
--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -1708,6 +1708,17 @@ protected:
    */
   void RecomputeFontSizeInflationEnabled();
 
+  /**
+   * Does the actual work of figuring out the current state of font size inflation.
+   */
+  bool DetermineFontSizeInflationState();
+
+  /**
+   * Apply the system font scale from the corresponding pref to the PresContext,
+   * taking into account the current state of font size inflation.
+   */
+  void HandleSystemFontScale();
+
   void RecordAlloc(void* aPtr) {
 #ifdef DEBUG
     MOZ_ASSERT(!mAllocatedPointers.Contains(aPtr));
@@ -1915,11 +1926,12 @@ protected:
   bool mFontSizeInflationForceEnabled;
   bool mFontSizeInflationDisabledInMasterProcess;
   bool mFontSizeInflationEnabled;
-  bool mPaintingIsFrozen;
 
   // Dirty bit indicating that mFontSizeInflationEnabled needs to be recomputed.
   bool mFontSizeInflationEnabledIsDirty;
 
+  bool mPaintingIsFrozen;
+
   // If a document belongs to an invisible DocShell, this flag must be set
   // to true, so we can avoid any paint calls for widget related to this
   // presshell.
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 0d516ccea7cc..488c258857a5 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -210,7 +210,9 @@ nsPresContext::nsPresContext(nsIDocument* aDocument, nsPresContextType aType)
     mLinkHandler(nullptr),
     mInflationDisabledForShrinkWrap(false),
     mBaseMinFontSize(0),
+    mSystemFontScale(1.0),
     mTextZoom(1.0),
+    mEffectiveTextZoom(1.0),
     mFullZoom(1.0),
     mOverrideDPPX(0.0),
     mLastFontInflationScreenSize(gfxSize(-1.0, -1.0)),
@@ -1318,6 +1320,29 @@ nsPresContext::GetContentLanguage() const
   return nullptr;
 }
 
+void
+nsPresContext::UpdateEffectiveTextZoom()
+{
+  float newZoom = mSystemFontScale * mTextZoom;
+  float minZoom = nsLayoutUtils::MinZoom();
+  float maxZoom = nsLayoutUtils::MaxZoom();
+
+  if (newZoom < minZoom) {
+    newZoom = minZoom;
+  } else if (newZoom > maxZoom) {
+    newZoom = maxZoom;
+  }
+
+  mEffectiveTextZoom = newZoom;
+
+  if (HasCachedStyleData()) {
+    // Media queries could have changed, since we changed the meaning
+    // of 'em' units in them.
+    MediaFeatureValuesChanged(eRestyle_ForceDescendants,
+                              NS_STYLE_HINT_REFLOW);
+  }
+}
+
 void
 nsPresContext::SetFullZoom(float aZoom)
 {
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index b1bcab1fae71..72272feddb96 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -535,6 +535,30 @@ public:
   nsIAtom* GetLanguageFromCharset() const { return mLanguage; }
   already_AddRefed<nsIAtom> GetContentLanguage() const;
 
+  /**
+   * Get/set a text zoom factor that is applied on top of the normal text zoom
+   * set by the front-end/user.
+   */
+  float GetSystemFontScale() const { return mSystemFontScale; }
+  void SetSystemFontScale(float aFontScale) {
+    MOZ_ASSERT(aFontScale > 0.0f, "invalid font scale");
+    if (aFontScale == mSystemFontScale || IsPrintingOrPrintPreview()) {
+      return;
+    }
+
+    mSystemFontScale = aFontScale;
+    UpdateEffectiveTextZoom();
+  }
+
+  /**
+   * Get/set the text zoom factor in use.
+   * This value should be used if you're interested in the pure text zoom value
+   * controlled by the front-end, e.g. when transferring zoom levels to a new
+   * document.
+   * Code that wants to use this value for layouting and rendering purposes
+   * should consider using EffectiveTextZoom() instead, so as to take the system
+   * font scale into account as well.
+   */
   float TextZoom() const { return mTextZoom; }
   void SetTextZoom(float aZoom) {
     MOZ_ASSERT(aZoom > 0.0f, "invalid zoom factor");
@@ -542,14 +566,23 @@ public:
       return;
 
     mTextZoom = aZoom;
-    if (HasCachedStyleData()) {
-      // Media queries could have changed, since we changed the meaning
-      // of 'em' units in them.
-      MediaFeatureValuesChanged(eRestyle_ForceDescendants,
-                                NS_STYLE_HINT_REFLOW);
-    }
+    UpdateEffectiveTextZoom();
   }
 
+protected:
+  void UpdateEffectiveTextZoom();
+
+public:
+  /**
+   * Corresponds to the product of text zoom and system font scale, limited
+   * by zoom.maxPercent and minPercent.
+   * As the system font scale is automatically set by the PresShell, code that
+   * e.g. wants to transfer zoom levels to a new document should use TextZoom()
+   * instead, which corresponds to the text zoom level that was actually set by
+   * the front-end/user.
+   */
+  float EffectiveTextZoom() const { return mEffectiveTextZoom; }
+
   /**
    * Get the minimum font size for the specified language. If aLanguage
    * is nullptr, then the document's language is used.  This combines
@@ -931,6 +964,7 @@ public:
   bool IsScreen() { return (mMedium == nsGkAtoms::screen ||
                               mType == eContext_PageLayout ||
                               mType == eContext_PrintPreview); }
+  bool IsPrintingOrPrintPreview() { return (mType == eContext_Print || mType == eContext_PrintPreview); }
 
   // Is this presentation in a chrome docshell?
   bool IsChrome() const { return mIsChrome; }
@@ -1300,7 +1334,9 @@ protected:
 
   // Base minimum font size, independent of the language-specific global preference. Defaults to 0
   int32_t               mBaseMinFontSize;
+  float                 mSystemFontScale; // Internal text zoom factor, defaults to 1.0
   float                 mTextZoom;      // Text zoom, defaults to 1.0
+  float                 mEffectiveTextZoom; // Text zoom * system font scale
   float                 mFullZoom;      // Page zoom, defaults to 1.0
   float                 mOverrideDPPX;   // DPPX overrided, defaults to 0.0
   gfxSize               mLastFontInflationScreenSize;
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index bc6dd26af9bf..59eb408fa40f 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -5414,7 +5414,7 @@ nsComputedDOMStyle::GetLineHeightCoord(nscoord& aCoord)
   const nsStyleFont* font = StyleFont();
   float fCoord = float(aCoord);
   if (font->mAllowZoom) {
-    fCoord /= mPresShell->GetPresContext()->TextZoom();
+    fCoord /= mPresShell->GetPresContext()->EffectiveTextZoom();
   }
   if (font->mFont.size != font->mSize) {
     fCoord = fCoord * (float(font->mSize) / float(font->mFont.size));
diff --git a/layout/style/nsStyleStruct.cpp b/layout/style/nsStyleStruct.cpp
index ceb6f9ac0dc6..1fe0fdba0b2b 100644
--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -200,7 +200,7 @@ nsStyleFont::ZoomText(const nsPresContext* aPresContext, nscoord aSize)
 {
   // aSize can be negative (e.g.: calc(-1px)) so we can't assert that here.
   // The caller is expected deal with that.
-  return NSToCoordTruncClamped(float(aSize) * aPresContext->TextZoom());
+  return NSToCoordTruncClamped(float(aSize) * aPresContext->EffectiveTextZoom());
 }
 
 /* static */ nscoord
@@ -208,7 +208,7 @@ nsStyleFont::UnZoomText(nsPresContext *aPresContext, nscoord aSize)
 {
   // aSize can be negative (e.g.: calc(-1px)) so we can't assert that here.
   // The caller is expected deal with that.
-  return NSToCoordTruncClamped(float(aSize) / aPresContext->TextZoom());
+  return NSToCoordTruncClamped(float(aSize) / aPresContext->EffectiveTextZoom());
 }
 
 /* static */ already_AddRefed<nsIAtom>