--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -6767,20 +6767,19 @@ nsViewportInfo Document::GetViewportInfo
       float extendZoom = ComputeExtendZoom();
 
       CSSCoord minWidth = mMinWidth;
       CSSCoord maxWidth = mMaxWidth;
       CSSCoord minHeight = mMinHeight;
       CSSCoord maxHeight = mMaxHeight;
 
       // aDisplaySize is in screen pixels; convert them to CSS pixels for the
-      // viewport size.
-      CSSToScreenScale defaultPixelScale =
-          layoutDeviceScale * LayoutDeviceToScreenScale(1.0f);
-      CSSSize displaySize = ScreenSize(aDisplaySize) / defaultPixelScale;
+      // viewport size. We need to use this scaled size for any clamping of
+      // width or height.
+      CSSSize displaySize = ScreenSize(aDisplaySize) / defaultScale;
 
       // Resolve device-width and device-height first.
       if (maxWidth == nsViewportInfo::DeviceSize) {
         maxWidth = displaySize.width;
       }
       if (maxHeight == nsViewportInfo::DeviceSize) {
         maxHeight = displaySize.height;
       }
@@ -6847,17 +6846,17 @@ nsViewportInfo Document::GetViewportInfo
           width = height * aDisplaySize.width / aDisplaySize.height;
         }
       }
 
       // Resolve height value
       // https://drafts.csswg.org/css-device-adapt/#resolve-height
       if (height == nsViewportInfo::Auto) {
         if (aDisplaySize.width == 0) {
-          height = aDisplaySize.height;
+          height = displaySize.height;
         } else {
           height = width * aDisplaySize.height / aDisplaySize.width;
         }
       }
       MOZ_ASSERT(width != nsViewportInfo::Auto &&
                  height != nsViewportInfo::Auto);
 
       CSSSize size(width, height);
@@ -6890,18 +6889,18 @@ nsViewportInfo Document::GetViewportInfo
       CSSSize effectiveMinSize = Min(CSSSize(kViewportMinSize), displaySize);
 
       size.width = clamped(size.width, effectiveMinSize.width,
                            float(kViewportMaxSize.width));
 
       // Also recalculate the default zoom, if it wasn't specified in the
       // metadata, and the width is specified.
       if (mScaleStrEmpty && !mWidthStrEmpty) {
-        CSSToScreenScale defaultScale(float(aDisplaySize.width) / size.width);
-        scaleFloat = (scaleFloat > defaultScale) ? scaleFloat : defaultScale;
+        CSSToScreenScale bestFitScale(float(aDisplaySize.width) / size.width);
+        scaleFloat = (scaleFloat > bestFitScale) ? scaleFloat : bestFitScale;
       }
 
       size.height = clamped(size.height, effectiveMinSize.height,
                             float(kViewportMaxSize.height));
 
       // We need to perform a conversion, but only if the initial or maximum
       // scale were set explicitly by the user.
       if (mValidScaleFloat && scaleFloat >= scaleMinFloat &&