--- a/layout/painting/nsCSSRenderingGradients.cpp
+++ b/layout/painting/nsCSSRenderingGradients.cpp
@@ -1062,17 +1062,18 @@ nsCSSGradientRenderer::BuildWebRenderDis
   LayoutDeviceRect gradientBounds = LayoutDeviceRect(firstTileBounds.TopLeft(),
                                                      LayoutDeviceSize(tileToClip.x, tileToClip.y));
 
   // Calculate the tile spacing, which is the repeat size minus the tile size
   LayoutDeviceSize tileSpacing = tileRepeat - firstTileBounds.Size();
 
   // Make the rects relative to the parent stacking context
   LayerRect layerClipBounds = aLayer->RelativeToParent(clipBounds);
-  LayerRect layerFirstTileBounds = aLayer->RelativeToParent(firstTileBounds);
+  LayerSize layerFirstTileSize = ViewAs<LayerPixel>(firstTileBounds.Size(),
+      PixelCastJustification::WebRenderHasUnitResolution);
   LayerRect layerGradientBounds = aLayer->RelativeToParent(gradientBounds);
 
   // srcTransform is used for scaling the gradient to match aSrc
   LayoutDeviceRect srcTransform = LayoutDeviceRect(mPresContext->CSSPixelsToAppUnits(aSrc.x),
                                                    mPresContext->CSSPixelsToAppUnits(aSrc.y),
                                                    aDest.width / ((float)mPresContext->CSSPixelsToAppUnits(aSrc.width)),
                                                    aDest.height / ((float)mPresContext->CSSPixelsToAppUnits(aSrc.height)));
 
@@ -1085,27 +1086,27 @@ nsCSSGradientRenderer::BuildWebRenderDis
 
     aBuilder.PushLinearGradient(
       mozilla::wr::ToWrRect(layerGradientBounds),
       aBuilder.BuildClipRegion(mozilla::wr::ToWrRect(layerClipBounds)),
       mozilla::wr::ToWrPoint(lineStart),
       mozilla::wr::ToWrPoint(lineEnd),
       stops,
       extendMode,
-      mozilla::wr::ToWrSize(layerFirstTileBounds.Size()),
+      mozilla::wr::ToWrSize(layerFirstTileSize),
       mozilla::wr::ToWrSize(tileSpacing));
   } else {
     gradientRadius.width *= srcTransform.width;
     gradientRadius.height *= srcTransform.height;
 
     aBuilder.PushRadialGradient(
       mozilla::wr::ToWrRect(layerGradientBounds),
       aBuilder.BuildClipRegion(mozilla::wr::ToWrRect(layerClipBounds)),
       mozilla::wr::ToWrPoint(lineStart),
       mozilla::wr::ToWrSize(gradientRadius),
       stops,
       extendMode,
-      mozilla::wr::ToWrSize(layerFirstTileBounds.Size()),
+      mozilla::wr::ToWrSize(layerFirstTileSize),
       mozilla::wr::ToWrSize(tileSpacing));
   }
 }
 
 } // namespace mozilla

--- a/layout/painting/nsImageRenderer.cpp
+++ b/layout/painting/nsImageRenderer.cpp
@@ -619,33 +619,32 @@ nsImageRenderer::BuildWebRenderDisplayIt
       Maybe<wr::ImageKey> key = aLayer->SendImageContainer(container, aParentCommands);
       if (key.isNothing()) {
         return DrawResult::BAD_IMAGE;
       }
 
       const int32_t appUnitsPerDevPixel = mForFrame->PresContext()->AppUnitsPerDevPixel();
       LayoutDeviceRect destRect = LayoutDeviceRect::FromAppUnits(
           aDest, appUnitsPerDevPixel);
-      LayerRect dest = aLayer->RelativeToParent(destRect);
 
       nsPoint firstTilePos = nsLayoutUtils::GetBackgroundFirstTilePos(aDest.TopLeft(),
                                                                       aFill.TopLeft(),
                                                                       aRepeatSize);
       LayoutDeviceRect fillRect = LayoutDeviceRect::FromAppUnits(
           nsRect(firstTilePos.x, firstTilePos.y,
                  aFill.XMost() - firstTilePos.x, aFill.YMost() - firstTilePos.y),
           appUnitsPerDevPixel);
       LayerRect fill = aLayer->RelativeToParent(fillRect);
       LayerRect clip = aLayer->RelativeToParent(
                                  LayoutDeviceRect::FromAppUnits(aFill,appUnitsPerDevPixel));
 
-      Size gapSize((aRepeatSize.width - aDest.width) / appUnitsPerDevPixel,
-                   (aRepeatSize.height - aDest.height) / appUnitsPerDevPixel);
+      LayoutDeviceSize gapSize = LayoutDeviceSize::FromAppUnits(
+          aRepeatSize - aDest.Size(), appUnitsPerDevPixel);
       aBuilder.PushImage(wr::ToWrRect(fill), aBuilder.BuildClipRegion(wr::ToWrRect(clip)),
-                         wr::ToWrSize(dest.Size()), wr::ToWrSize(gapSize),
+                         wr::ToWrSize(destRect.Size()), wr::ToWrSize(gapSize),
                          wr::ImageRendering::Auto, key.value());
       break;
     }
     default:
       break;
   }
 
   return DrawResult::SUCCESS;