--- a/content/base/public/nsContentUtils.h
+++ b/content/base/public/nsContentUtils.h
@@ -1544,16 +1544,21 @@ public:
    *
    * NOTE: If the site is optimized for mobile (via the doctype), this
    * will return viewport information that specifies default information.
    */
   static ViewportInfo GetViewportInfo(nsIDocument* aDocument,
                                       uint32_t aDisplayWidth,
                                       uint32_t aDisplayHeight);
 
+  /**
+   * The device-pixel-to-CSS-px ratio used to adjust meta viewport values.
+   */
+  static double GetDevicePixelsPerMetaViewportPixel(nsIWidget* aWidget);
+
   // Call EnterMicroTask when you're entering JS execution.
   // Usually the best way to do this is to use nsAutoMicroTask.
   static void EnterMicroTask() { ++sMicroTaskLevel; }
   static void LeaveMicroTask();
 
   static bool IsInMicroTask() { return sMicroTaskLevel != 0; }
   static uint32_t MicroTaskLevel() { return sMicroTaskLevel; }
   static void SetMicroTaskLevel(uint32_t aLevel) { sMicroTaskLevel = aLevel; }

--- a/content/base/src/nsContentUtils.cpp
+++ b/content/base/src/nsContentUtils.cpp
@@ -7,16 +7,18 @@
 /* A namespace class for static layout utilities. */
 
 #include "mozilla/Util.h"
 
 #include "jsapi.h"
 #include "jsdbgapi.h"
 #include "jsfriendapi.h"
 
+#include <math.h>
+
 #include "Layers.h"
 #include "nsJSUtils.h"
 #include "nsCOMPtr.h"
 #include "nsAString.h"
 #include "nsPrintfCString.h"
 #include "nsIScriptGlobalObject.h"
 #include "nsIScriptContext.h"
 #include "nsDOMCID.h"
@@ -5169,44 +5171,64 @@ nsContentUtils::GetViewportInfo(nsIDocum
 
   if (widthStr.IsEmpty() &&
      (heightStr.EqualsLiteral("device-height") ||
           scaleFloat == 1.0))
   {
     autoSize = true;
   }
 
-  uint32_t width = widthStr.ToInteger(&errorCode);
-  if (NS_FAILED(errorCode)) {
-    if (autoSize) {
-      width = aDisplayWidth;
-    } else {
-      width = Preferences::GetInt("browser.viewport.desktopWidth", 0);
+  // Now convert the scale into device pixels per CSS pixel.
+  nsIWidget *widget = WidgetForDocument(aDocument);
+  double pixelRatio = widget ? GetDevicePixelsPerMetaViewportPixel(widget) : 1.0;
+  scaleFloat *= pixelRatio;
+  scaleMinFloat *= pixelRatio;
+  scaleMaxFloat *= pixelRatio;
+
+  uint32_t width, height;
+  if (autoSize) {
+    // aDisplayWidth and aDisplayHeight are in device pixels; convert them to
+    // CSS pixels for the viewport size.
+    width = aDisplayWidth / pixelRatio;
+    height = aDisplayHeight / pixelRatio;
+  } else {
+    nsresult widthErrorCode, heightErrorCode;
+    width = widthStr.ToInteger(&widthErrorCode);
+    height = heightStr.ToInteger(&heightErrorCode);
+
+    // If width or height has not been set to a valid number by this point,
+    // fall back to a default value.
+    bool validWidth = (!widthStr.IsEmpty() && NS_SUCCEEDED(widthErrorCode) && width > 0);
+    bool validHeight = (!heightStr.IsEmpty() && NS_SUCCEEDED(heightErrorCode) && height > 0);
+
+    if (!validWidth) {
+      if (validHeight && aDisplayWidth > 0 && aDisplayHeight > 0) {
+        width = uint32_t((height * aDisplayWidth) / aDisplayHeight);
+      } else {
+        width = Preferences::GetInt("browser.viewport.desktopWidth",
+                                    kViewportDefaultScreenWidth);
+      }
+    }
+
+    if (!validHeight) {
+      if (aDisplayWidth > 0 && aDisplayHeight > 0) {
+        height = uint32_t((width * aDisplayHeight) / aDisplayWidth);
+      } else {
+        height = width;
+      }
     }
   }
 
   width = NS_MIN(width, kViewportMaxWidth);
   width = NS_MAX(width, kViewportMinWidth);
 
   // Also recalculate the default zoom, if it wasn't specified in the metadata,
   // and the width is specified.
   if (scaleStr.IsEmpty() && !widthStr.IsEmpty()) {
-    scaleFloat = NS_MAX(scaleFloat, (float)(aDisplayWidth/width));
-  }
-
-  uint32_t height = heightStr.ToInteger(&errorCode);
-
-  if (NS_FAILED(errorCode)) {
-    height = width * ((float)aDisplayHeight / aDisplayWidth);
-  }
-
-  // If height was provided by the user, but width wasn't, then we should
-  // calculate the width.
-  if (widthStr.IsEmpty() && !heightStr.IsEmpty()) {
-    width = (uint32_t) ((height * aDisplayWidth) / aDisplayHeight);
+    scaleFloat = NS_MAX(scaleFloat, float(aDisplayWidth) / float(width));
   }
 
   height = NS_MIN(height, kViewportMaxHeight);
   height = NS_MAX(height, kViewportMinHeight);
 
   // We need to perform a conversion, but only if the initial or maximum
   // scale were set explicitly by the user.
   if (!scaleStr.IsEmpty() && NS_SUCCEEDED(scaleErrorCode)) {
@@ -5233,16 +5255,38 @@ nsContentUtils::GetViewportInfo(nsIDocum
   ret.defaultZoom = scaleFloat;
   ret.minZoom = scaleMinFloat;
   ret.maxZoom = scaleMaxFloat;
   ret.autoSize = autoSize;
   return ret;
 }
 
 /* static */
+double
+nsContentUtils::GetDevicePixelsPerMetaViewportPixel(nsIWidget* aWidget)
+{
+  int32_t prefValue = Preferences::GetInt("browser.viewport.scaleRatio", 0);
+  if (prefValue > 0) {
+    return double(prefValue) / 100.0;
+  }
+
+  float dpi = aWidget->GetDPI();
+  if (dpi < 200.0) {
+    // Includes desktop displays, LDPI and MDPI Android devices
+    return 1.0;
+  }
+  if (dpi < 300.0) {
+    // Includes Nokia N900, and HDPI Android devices
+    return 1.5;
+  }
+  // For very high-density displays like the iPhone 4, use an integer ratio.
+  return floor(dpi / 150.0);
+}
+
+/* static */
 nsresult
 nsContentUtils::ProcessViewportInfo(nsIDocument *aDocument,
                                     const nsAString &viewportInfo) {
 
   /* We never fail. */
   nsresult rv = NS_OK;
 
   aDocument->SetHeaderData(nsGkAtoms::viewport, viewportInfo);