--- a/layout/base/ShapeUtils.cpp
+++ b/layout/base/ShapeUtils.cpp
@@ -4,16 +4,17 @@
  * 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 "mozilla/ShapeUtils.h"
 
 #include <cstdlib>
 
 #include "nsCSSRendering.h"
+#include "nsMargin.h"
 #include "nsRuleNode.h"
 #include "nsStyleCoord.h"
 #include "nsStyleStruct.h"
 #include "SVGContentUtils.h"
 
 namespace mozilla {
 
 nscoord
@@ -110,9 +111,30 @@ ShapeUtils::ComputeEllipseRadii(StyleBas
                                       aRefBox.YMost());
   } else {
     radii.height = nsRuleNode::ComputeCoordPercentCalc(coords[1], aRefBox.height);
   }
 
   return radii;
 }
 
+/* static */ nsRect
+ShapeUtils::ComputeInsetRect(StyleBasicShape* const aBasicShape,
+                             const nsRect& aRefBox)
+{
+  MOZ_ASSERT(aBasicShape->GetShapeType() == StyleBasicShapeType::Inset,
+             "The basic shape must be inset()!");
+
+  const nsTArray<nsStyleCoord>& coords = aBasicShape->Coordinates();
+  MOZ_ASSERT(coords.Length() == 4, "wrong number of arguments");
+
+  nsMargin inset(nsRuleNode::ComputeCoordPercentCalc(coords[0], aRefBox.height),
+                 nsRuleNode::ComputeCoordPercentCalc(coords[1], aRefBox.width),
+                 nsRuleNode::ComputeCoordPercentCalc(coords[2], aRefBox.height),
+                 nsRuleNode::ComputeCoordPercentCalc(coords[3], aRefBox.width));
+
+  nsRect insetRect(aRefBox);
+  insetRect.Deflate(inset);
+
+  return insetRect;
+}
+
 } // namespace mozilla

--- a/layout/base/ShapeUtils.h
+++ b/layout/base/ShapeUtils.h
@@ -21,25 +21,23 @@ class StyleBasicShape;
 // https://drafts.csswg.org/css-shapes/#basic-shape-functions
 //
 struct ShapeUtils final
 {
   // Compute the length of a keyword <shape-radius>, i.e. closest-side or
   // farthest-side, for a circle or an ellipse on a single dimension. The
   // caller needs to call for both dimensions and combine the result.
   // https://drafts.csswg.org/css-shapes/#typedef-shape-radius.
-  //
   // @return The length of the radius in app units.
   static nscoord ComputeShapeRadius(const StyleShapeRadius aType,
                                     const nscoord aCenter,
                                     const nscoord aPosMin,
                                     const nscoord aPosMax);
 
   // Compute the center of a circle or an ellipse.
-  //
   // @param aRefBox The reference box of the basic shape.
   // @return The point of the center.
   static nsPoint ComputeCircleOrEllipseCenter(
     StyleBasicShape* const aBasicShape,
     const nsRect& aRefBox);
 
   // Compute the radius for a circle.
   // @param aCenter the center of the circle.
@@ -52,13 +50,20 @@ struct ShapeUtils final
   // Compute the radii for an ellipse.
   // @param aCenter the center of the ellipse.
   // @param aRefBox the reference box of the ellipse.
   // @return The radii of the ellipse in app units. The width and height
   // represent the x-axis and y-axis radii of the ellipse.
   static nsSize ComputeEllipseRadii(
     mozilla::StyleBasicShape* const aBasicShape,
     const nsPoint& aCenter, const nsRect& aRefBox);
+
+  // Compute the rect for an inset.
+  // @param aRefBox the reference box of the inset.
+  // @return The inset rect in app units.
+  static nsRect ComputeInsetRect(
+    mozilla::StyleBasicShape* const aBasicShape,
+    const nsRect& aRefBox);
 };
 
 } // namespace mozilla
 
 #endif // mozilla_ShapeUtils_h

--- a/layout/svg/nsCSSClipPathInstance.cpp
+++ b/layout/svg/nsCSSClipPathInstance.cpp
@@ -172,31 +172,23 @@ nsCSSClipPathInstance::CreateClipPathPol
   return builder->Finish();
 }
 
 already_AddRefed<Path>
 nsCSSClipPathInstance::CreateClipPathInset(DrawTarget* aDrawTarget,
                                            const nsRect& aRefBox)
 {
   StyleBasicShape* basicShape = mClipPathStyle.GetBasicShape();
-  const nsTArray<nsStyleCoord>& coords = basicShape->Coordinates();
-  MOZ_ASSERT(coords.Length() == 4, "wrong number of arguments");
 
   RefPtr<PathBuilder> builder = aDrawTarget->CreatePathBuilder();
 
   nscoord appUnitsPerDevPixel =
     mTargetFrame->PresContext()->AppUnitsPerDevPixel();
 
-  nsMargin inset(nsRuleNode::ComputeCoordPercentCalc(coords[0], aRefBox.height),
-                 nsRuleNode::ComputeCoordPercentCalc(coords[1], aRefBox.width),
-                 nsRuleNode::ComputeCoordPercentCalc(coords[2], aRefBox.height),
-                 nsRuleNode::ComputeCoordPercentCalc(coords[3], aRefBox.width));
-
-  nsRect insetRect(aRefBox);
-  insetRect.Deflate(inset);
+  nsRect insetRect = ShapeUtils::ComputeInsetRect(basicShape, aRefBox);
   const Rect insetRectPixels = NSRectToRect(insetRect, appUnitsPerDevPixel);
   const nsStyleCorners& radius = basicShape->GetRadius();
 
   nscoord appUnitsRadii[8];
 
   if (nsIFrame::ComputeBorderRadii(radius, insetRect.Size(), aRefBox.Size(),
                                    Sides(), appUnitsRadii)) {
     RectCornerRadii corners;