--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -1439,19 +1439,16 @@ nsIDocument::nsIDocument()
     mDOMCompleteSet(false),
     mAutoFocusFired(false),
     mScrolledToRefAlready(false),
     mChangeScrollPosWhenScrollingToRef(false),
     mHasWarnedAboutBoxObjects(false),
     mDelayFrameLoaderInitialization(false),
     mSynchronousDOMContentLoaded(false),
     mMaybeServiceWorkerControlled(false),
-    mValidWidth(false),
-    mValidHeight(false),
-    mAutoSize(false),
     mAllowZoom(false),
     mValidScaleFloat(false),
     mValidMaxScale(false),
     mScaleStrEmpty(false),
     mWidthStrEmpty(false),
     mParserAborted(false),
     mReportedUseCounters(false),
     mHasReportedShadowDOMUsage(false),
@@ -7432,38 +7429,16 @@ nsIDocument::GetViewportInfo(const Scree
 
     GetHeaderData(nsGkAtoms::viewport_height, heightStr);
     GetHeaderData(nsGkAtoms::viewport_width, widthStr);
 
     // Parse width and height properties
     // This function sets m{Min,Max}{Width,Height}.
     ParseWidthAndHeightInMetaViewport(widthStr, heightStr, scaleStr);
 
-    mAutoSize = false;
-    if (mMaxWidth == nsViewportInfo::DeviceSize) {
-      mAutoSize = true;
-    }
-    if (widthStr.IsEmpty() &&
-        (mMaxHeight == nsViewportInfo::DeviceSize ||
-         (mScaleFloat.scale == 1.0)))
-    {
-      mAutoSize = true;
-    }
-
-    // If width or height has not been set to a valid number by this point,
-    // fall back to a default value.
-    mValidWidth = (!widthStr.IsEmpty() && mMaxWidth > 0);
-    mValidHeight = (!heightStr.IsEmpty() && mMaxHeight > 0);
-
-    // If the width is set to some unrecognized value, and there is no
-    // height set, treat it as if device-width were specified.
-    if ((!mValidWidth && !widthStr.IsEmpty()) && !mValidHeight) {
-      mAutoSize = true;
-    }
-
     mAllowZoom = true;
     nsAutoString userScalable;
     GetHeaderData(nsGkAtoms::viewport_user_scalable, userScalable);
 
     if ((userScalable.EqualsLiteral("0")) ||
         (userScalable.EqualsLiteral("no")) ||
         (userScalable.EqualsLiteral("false"))) {
       mAllowZoom = false;
@@ -7603,17 +7578,26 @@ nsIDocument::GetViewportInfo(const Scree
     MOZ_ASSERT(width != nsViewportInfo::Auto && height != nsViewportInfo::Auto);
 
     CSSSize size(width, height);
 
     CSSToScreenScale scaleFloat = mScaleFloat * layoutDeviceScale;
     CSSToScreenScale scaleMinFloat = effectiveMinScale * layoutDeviceScale;
     CSSToScreenScale scaleMaxFloat = effectiveMaxScale * layoutDeviceScale;
 
-    if (mAutoSize) {
+    const bool autoSize =
+      mMaxWidth == nsViewportInfo::DeviceSize ||
+      (mWidthStrEmpty &&
+       (mMaxHeight == nsViewportInfo::DeviceSize ||
+        mScaleFloat.scale == 1.0f)) ||
+      (!mWidthStrEmpty && mMaxWidth == nsViewportInfo::Auto && mMaxHeight < 0);
+
+    // FIXME: Resolving width and height should be done above 'Resolve width
+    // value' and 'Resolve height value'.
+    if (autoSize) {
       size = displaySize;
     }
 
     size.width = clamped(size.width, float(kViewportMinSize.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) {
@@ -7631,17 +7615,17 @@ nsIDocument::GetViewportInfo(const Scree
       size.height = std::max(size.height, displaySize.height);
     } else if (effectiveValidMaxScale) {
       CSSSize displaySize = ScreenSize(aDisplaySize) / scaleMaxFloat;
       size.width = std::max(size.width, displaySize.width);
       size.height = std::max(size.height, displaySize.height);
     }
 
     return nsViewportInfo(scaleFloat, scaleMinFloat, scaleMaxFloat, size,
-                          mAutoSize, effectiveAllowZoom);
+                          autoSize, effectiveAllowZoom);
   }
 }
 
 void
 nsIDocument::UpdateViewportOverflowType(nscoord aScrolledWidth,
                                         nscoord aScrollportWidth)
 {
 #ifdef DEBUG

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -4313,19 +4313,16 @@ protected:
   bool mSynchronousDOMContentLoaded: 1;
 
   // Set to true when the document is possibly controlled by the ServiceWorker.
   // Used to prevent multiple requests to ServiceWorkerManager.
   bool mMaybeServiceWorkerControlled: 1;
 
   // These member variables cache information about the viewport so we don't
   // have to recalculate it each time.
-  bool mValidWidth: 1;
-  bool mValidHeight: 1;
-  bool mAutoSize: 1;
   bool mAllowZoom: 1;
   bool mValidScaleFloat: 1;
   bool mValidMaxScale: 1;
   bool mScaleStrEmpty: 1;
   bool mWidthStrEmpty: 1;
 
   // Parser aborted. True if the parser of this document was forcibly
   // terminated instead of letting it finish at its own pace.