--- a/dom/svg/SVGSVGElement.cpp
+++ b/dom/svg/SVGSVGElement.cpp
@@ -2,16 +2,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include <stdint.h>
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/ContentEvents.h"
 #include "mozilla/EventDispatcher.h"
+#include "mozilla/FloatingPoint.h"
 #include "mozilla/Likely.h"
 
 #include "nsGkAtoms.h"
 #include "nsLayoutUtils.h"
 #include "nsLayoutStylesheetCache.h"
 #include "DOMSVGNumber.h"
 #include "DOMSVGLength.h"
 #include "nsSVGAngle.h"
@@ -1242,10 +1243,36 @@ SVGSVGElement::GetTransformProperty() co
 }
 
 bool
 SVGSVGElement::ClearTransformProperty()
 {
   return UnsetProperty(nsGkAtoms::transform);
 }
 
+float
+SVGSVGElement::GetIntrinsicWidth()
+{
+  if (mLengthAttributes[ATTR_WIDTH].IsPercentage()) {
+    return UnspecifiedNaN<float>();
+  }
+  // Passing |this| as a SVGSVGElement* invokes the variant of GetAnimValue
+  // that uses the passed argument as the context, but that's fine since we
+  // know the length isn't a percentage so the context won't be used (and we
+  // need to pass the element to be able to resolve em/ex units).
+  return std::max(mLengthAttributes[ATTR_WIDTH].GetAnimValue(this), 0.f);
+}
+
+float
+SVGSVGElement::GetIntrinsicHeight()
+{
+  if (mLengthAttributes[ATTR_HEIGHT].IsPercentage()) {
+    return UnspecifiedNaN<float>();
+  }
+  // Passing |this| as a SVGSVGElement* invokes the variant of GetAnimValue
+  // that uses the passed argument as the context, but that's fine since we
+  // know the length isn't a percentage so the context won't be used (and we
+  // need to pass the element to be able to resolve em/ex units).
+  return std::max(mLengthAttributes[ATTR_HEIGHT].GetAnimValue(this), 0.f);
+}
+
 } // namespace dom
 } // namespace mozilla

--- a/dom/svg/SVGSVGElement.h
+++ b/dom/svg/SVGSVGElement.h
@@ -143,16 +143,24 @@ public:
   virtual bool HasValidDimensions() const MOZ_OVERRIDE;
 
   // SVGSVGElement methods:
   float GetLength(uint8_t mCtxType);
 
   // public helpers:
 
   /**
+   * Returns the user-unit width/height if those dimensions are not specified
+   * as percentage values. If they are specified as percentage values then this
+   * element does not have intrinsic width/height and these methods return NaN.
+   */
+  float GetIntrinsicWidth();
+  float GetIntrinsicHeight();
+
+  /**
    * Returns true if this element has a base/anim value for its "viewBox"
    * attribute that defines a viewBox rectangle with finite values, or
    * if there is a view element overriding this element's viewBox and it
    * has a valid viewBox.
    *
    * Note that this does not check whether we need to synthesize a viewBox,
    * so you must call ShouldSynthesizeViewBox() if you need to check that too.
    *

--- a/image/src/SVGDocumentWrapper.cpp
+++ b/image/src/SVGDocumentWrapper.cpp
@@ -1,16 +1,17 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "SVGDocumentWrapper.h"
 
 #include "mozilla/dom/Element.h"
+#include "mozilla/FloatingPoint.h"
 #include "nsICategoryManager.h"
 #include "nsIChannel.h"
 #include "nsIContentViewer.h"
 #include "nsIDocument.h"
 #include "nsIDocumentLoaderFactory.h"
 #include "nsIDOMSVGLength.h"
 #include "nsIHttpChannel.h"
 #include "nsIObserverService.h"
@@ -71,44 +72,24 @@ SVGDocumentWrapper::DestroyViewer()
 bool
 SVGDocumentWrapper::GetWidthOrHeight(Dimension aDimension,
                                      int32_t& aResult)
 {
   SVGSVGElement* rootElem = GetRootSVGElem();
   NS_ABORT_IF_FALSE(rootElem, "root elem missing or of wrong type");
 
   // Get the width or height SVG object
-  nsRefPtr<SVGAnimatedLength> domAnimLength;
-  if (aDimension == eWidth) {
-    domAnimLength = rootElem->Width();
-  } else {
-    NS_ABORT_IF_FALSE(aDimension == eHeight, "invalid dimension");
-    domAnimLength = rootElem->Height();
-  }
-  NS_ENSURE_TRUE(domAnimLength, false);
+  float length = (aDimension == eWidth) ? rootElem->GetIntrinsicWidth()
+                                        : rootElem->GetIntrinsicHeight();
 
-  // Get the animated value from the object
-  nsRefPtr<DOMSVGLength> domLength = domAnimLength->AnimVal();
-  NS_ENSURE_TRUE(domLength, false);
-
-  // Check if it's a percent value (and fail if so)
-  uint16_t unitType;
-  nsresult rv = domLength->GetUnitType(&unitType);
-  NS_ENSURE_SUCCESS(rv, false);
-  if (unitType == nsIDOMSVGLength::SVG_LENGTHTYPE_PERCENTAGE) {
-    return false;
+  if (!IsFinite(length)) {
+    return false; // Percentage size
   }
 
-  // Non-percent value - woot! Grab it & return it.
-  float floatLength;
-  rv = domLength->GetValue(&floatLength);
-  NS_ENSURE_SUCCESS(rv, false);
-
-  aResult = nsSVGUtils::ClampToInt(floatLength);
-
+  aResult = nsSVGUtils::ClampToInt(length);
   return true;
 }
 
 nsIFrame*
 SVGDocumentWrapper::GetRootLayoutFrame()
 {
   Element* rootElem = GetRootSVGElem();
   return rootElem ? rootElem->GetPrimaryFrame() : nullptr;