Merge branch 'master' of https://repo.palemoon.org/MoonchildProductions/UXP

30 files changed, 2621 insertions, 123 deletions

diff --git a/ipc/chromium/src/third_party/libevent/arc4random.c b/ipc/chromium/src/third_party/libevent/arc4random.c
index 0dd6934ef8..9efcc6e934 100644
--- a/ipc/chromium/src/third_party/libevent/arc4random.c
+++ b/ipc/chromium/src/third_party/libevent/arc4random.c
@@ -481,7 +481,7 @@ arc4random(void)
 }
 #endif
 
-#if defined(_we_have_arc4random_buf) || !defined(XP_SOLARIS)
+#ifndef HAVE_ARC4RANDOM_BUF
 ARC4RANDOM_EXPORT void
 arc4random_buf(void *_buf, size_t n)
 {
diff --git a/layout/generic/ReflowInput.cpp b/layout/generic/ReflowInput.cpp
index 1af7e798ef..994e2cfbb6 100644
--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -225,7 +225,7 @@ ReflowInput::ReflowInput(
     CheckNextInFlowParenthood(aFrame, aParentReflowInput.mFrame);
   mFlags.mAssumingHScrollbar = mFlags.mAssumingVScrollbar = false;
   mFlags.mIsColumnBalancing = false;
-  mFlags.mIsFlexContainerMeasuringHeight = false;
+  mFlags.mIsFlexContainerMeasuringBSize = false;
   mFlags.mDummyParentReflowInput = false;
   mFlags.mShrinkWrap = !!(aFlags & COMPUTE_SIZE_SHRINK_WRAP);
   mFlags.mUseAutoBSize = !!(aFlags & COMPUTE_SIZE_USE_AUTO_BSIZE);
@@ -2403,15 +2403,15 @@ ReflowInput::InitConstraints(nsPresContext*     aPresContext,
             ComputeSizeFlags(computeSizeFlags | ComputeSizeFlags::eShrinkWrap);
 
           // If we're inside of a flex container that needs to measure our
-          // auto height, pass that information along to ComputeSize().
-          if (mFlags.mIsFlexContainerMeasuringHeight) {
+          // auto BSize, pass that information along to ComputeSize().
+          if (mFlags.mIsFlexContainerMeasuringBSize) {
             computeSizeFlags =
               ComputeSizeFlags(computeSizeFlags | ComputeSizeFlags::eUseAutoBSize);
           }
         } else {
-          MOZ_ASSERT(!mFlags.mIsFlexContainerMeasuringHeight,
+          MOZ_ASSERT(!mFlags.mIsFlexContainerMeasuringBSize,
                      "We're not in a flex container, so the flag "
-                     "'mIsFlexContainerMeasuringHeight' shouldn't be set");
+                     "'mIsFlexContainerMeasuringBSize' shouldn't be set");
         }
       }
 
@@ -2993,7 +2993,7 @@ ReflowInput::ComputeMinMaxValues(const LogicalSize&aCBSize)
        minBSize.HasPercent()) ||
       (mFrameType == NS_CSS_FRAME_TYPE_INTERNAL_TABLE &&
        minBSize.IsCalcUnit() && minBSize.CalcHasPercent()) ||
-      mFlags.mIsFlexContainerMeasuringHeight) {
+      mFlags.mIsFlexContainerMeasuringBSize) {
     ComputedMinBSize() = 0;
   } else {
     ComputedMinBSize() = ComputeBSizeValue(aCBSize.BSize(wm),
@@ -3015,7 +3015,7 @@ ReflowInput::ComputeMinMaxValues(const LogicalSize&aCBSize)
          maxBSize.HasPercent()) ||
         (mFrameType == NS_CSS_FRAME_TYPE_INTERNAL_TABLE &&
          maxBSize.IsCalcUnit() && maxBSize.CalcHasPercent()) ||
-        mFlags.mIsFlexContainerMeasuringHeight) {
+        mFlags.mIsFlexContainerMeasuringBSize) {
       ComputedMaxBSize() = NS_UNCONSTRAINEDSIZE;
     } else {
       ComputedMaxBSize() = ComputeBSizeValue(aCBSize.BSize(wm),
diff --git a/layout/generic/ReflowInput.h b/layout/generic/ReflowInput.h
index a70549d8eb..e84d386a82 100644
--- a/layout/generic/ReflowInput.h
+++ b/layout/generic/ReflowInput.h
@@ -203,9 +203,9 @@ public:
     uint32_t mHeightDependsOnAncestorCell:1;   // Does frame height depend on
                                                // an ancestor table-cell?
     uint32_t mIsColumnBalancing:1;   // nsColumnSetFrame is balancing columns
-    uint32_t mIsFlexContainerMeasuringHeight:1; // nsFlexContainerFrame is
-                                                // reflowing this child to
-                                                // measure its intrinsic height.
+    uint32_t mIsFlexContainerMeasuringBSize:1; // nsFlexContainerFrame is
+                                               // reflowing this child to
+                                               // measure its intrinsic BSize.
     uint32_t mDummyParentReflowInput:1; // a "fake" reflow state made
                                         // in order to be the parent
                                         // of a real one
diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
index 18a0643f14..b4907a4862 100644
--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -511,6 +511,15 @@ public:
   // visibility:collapse.
   bool IsStrut() const             { return mIsStrut; }
 
+  // Returns true if this item's inline axis is parallel (or antiparallel)
+  // to the container's main axis. Otherwise (i.e. if this item's inline axis
+  // is orthogonal to the container's main axis), this function returns false.
+  bool IsInlineAxisMainAxis() const { return mIsInlineAxisMainAxis; }
+
+  // Same as above, but for cross axis. Equivalent to !IsInlineAxisMainAxis().
+  // This just exists for convenience/readability at callsites.
+  bool IsInlineAxisCrossAxis() const { return !mIsInlineAxisMainAxis; }
+
   WritingMode GetWritingMode() const { return mWM; }
   uint8_t GetAlignSelf() const     { return mAlignSelf; }
 
@@ -749,10 +758,8 @@ protected:
   void CheckForMinSizeAuto(const ReflowInput& aFlexItemReflowInput,
                            const FlexboxAxisTracker& aAxisTracker);
 
-  // Our frame:
-  nsIFrame* const mFrame;
-
-  // Values that we already know in constructor: (and are hence mostly 'const')
+  // Values that we already know in constructor (and are hence mostly 'const'):
+  nsIFrame* const mFrame; // The flex item's frame.
   const float mFlexGrow;
   const float mFlexShrink;
 
@@ -785,6 +792,7 @@ protected:
   // until after main-size has been resolved. In particular, these could share
   // memory with mMainPosn through mAscent, and mIsStretched.
   float mShareOfWeightSoFar;
+  const WritingMode mWM; // The flex item's writing mode.
   bool mIsFrozen;
   bool mHadMinViolation;
   bool mHadMaxViolation;
@@ -795,11 +803,11 @@ protected:
   bool mIsStretched; // See IsStretched() documentation
   bool mIsStrut;     // Is this item a "strut" left behind by an element
                      // with visibility:collapse?
+  const bool mIsInlineAxisMainAxis; // See IsInlineAxisMainAxis() documentation
 
   // Does this item need to resolve a min-[width|height]:auto (in main-axis).
   bool mNeedsMinSizeAutoResolution;
 
-  const WritingMode mWM; // The flex item's writing mode.
   uint8_t mAlignSelf; // My "align-self" computed value (with "auto"
                       // swapped out for parent"s "align-items" value,
                       // in our constructor).
@@ -1492,21 +1500,26 @@ nsFlexContainerFrame::GenerateFlexItemForChild(
 
 // Static helper-functions for ResolveAutoFlexBasisAndMinSize():
 // -------------------------------------------------------------
-// Indicates whether the cross-size property is set to something definite.
-// The logic here should be similar to the logic for isAutoWidth/isAutoHeight
+// Indicates whether the cross-size property is set to something definite,
+// for the purpose of intrinsic ratio calculations.
+// The logic here should be similar to the logic for isAutoISize/isAutoBSize
 // in nsFrame::ComputeSizeWithIntrinsicDimensions().
 static bool
 IsCrossSizeDefinite(const ReflowInput& aItemReflowInput,
                     const FlexboxAxisTracker& aAxisTracker)
 {
   const nsStylePosition* pos = aItemReflowInput.mStylePosition;
-  if (aAxisTracker.IsCrossAxisHorizontal()) {
-    return pos->mWidth.GetUnit() != eStyleUnit_Auto;
-  }
-  // else, vertical. (We need to use IsAutoHeight() to catch e.g. %-height
-  // applied to indefinite-height containing block, which counts as auto.)
-  nscoord cbHeight = aItemReflowInput.mCBReflowInput->ComputedHeight();
-  return !nsLayoutUtils::IsAutoHeight(pos->mHeight, cbHeight);
+  const WritingMode containerWM = aAxisTracker.GetWritingMode();
+
+  if (aAxisTracker.IsColumnOriented()) {
+    // Column-oriented means cross axis is container's inline axis.
+    return pos->ISize(containerWM).GetUnit() != eStyleUnit_Auto;
+  }
+  // Else, we're row-oriented, which means cross axis is container's block
+  // axis. We need to use IsAutoBSize() to catch e.g. %-BSize applied to
+  // indefinite container BSize, which counts as auto.
+  nscoord cbBSize = aItemReflowInput.mCBReflowInput->ComputedBSize();
+  return !nsLayoutUtils::IsAutoBSize(pos->BSize(containerWM), cbBSize);
 }
 
 // If aFlexItem has a definite cross size, this function returns it, for usage
@@ -1676,9 +1689,10 @@ nsFlexContainerFrame::
                                  const ReflowInput& aItemReflowInput,
                                  const FlexboxAxisTracker& aAxisTracker)
 {
-  // (Note: We should never have a used flex-basis of "auto" if our main axis
-  // is horizontal; width values should always be resolvable without reflow.)
-  const bool isMainSizeAuto = (!aAxisTracker.IsMainAxisHorizontal() &&
+  // (Note: We can guarantee that the flex-basis will have already been
+  // resolved if the main axis is the same is the same as the item's inline
+  // axis. Inline-axis values should always be resolvable without reflow.)
+  const bool isMainSizeAuto = (!aFlexItem.IsInlineAxisMainAxis() &&
                                NS_AUTOHEIGHT == aFlexItem.GetFlexBaseSize());
 
   const bool isMainMinSizeAuto = aFlexItem.NeedsMinSizeAutoResolution();
@@ -1709,10 +1723,14 @@ nsFlexContainerFrame::
                                   flexContainerRI->ComputedISize(),
                                   flexContainerRI->ComputedBSize());
     // Is container's cross size "definite"?
-    // (Container's cross size is definite if cross-axis is horizontal, or if
-    // cross-axis is vertical and the cross-size is not NS_AUTOHEIGHT.)
-    if (aAxisTracker.IsCrossAxisHorizontal() ||
+    // - If it's column-oriented, then "yes", because its cross size is its
+    // inline-size which is always definite from its descendants' perspective.
+    // - Otherwise (if it's row-oriented), then we check the actual size
+    // and call it definite if it's not NS_AUTOHEIGHT.
+    if (aAxisTracker.IsColumnOriented() ||
         containerCrossSize != NS_AUTOHEIGHT) {
+      // Container's cross size is "definite", so we can resolve the item's
+      // stretched cross size using that.
       aFlexItem.ResolveStretchedCrossSize(containerCrossSize, aAxisTracker);
     }
   }
@@ -1744,7 +1762,7 @@ nsFlexContainerFrame::
 
   // Measure content, if needed (w/ intrinsic-width method or a reflow)
   if (minSizeNeedsToMeasureContent || flexBasisNeedsToMeasureContent) {
-    if (aAxisTracker.IsMainAxisHorizontal()) {
+    if (aFlexItem.IsInlineAxisMainAxis()) {
       if (minSizeNeedsToMeasureContent) {
         nscoord frameMinISize =
           aFlexItem.Frame()->GetMinISize(aItemReflowInput.mRenderingContext);
@@ -1755,30 +1773,32 @@ nsFlexContainerFrame::
                    "reflow state, for horizontal flexbox. It shouldn't need "
                    "special handling here");
     } else {
-      // If this item is flexible (vertically), or if we're measuring the
-      // 'auto' min-height and our main-size is something else, then we assume
-      // that the computed-height we're reflowing with now could be different
-      // from the one we'll use for this flex item's "actual" reflow later on.
-      // In that case, we need to be sure the flex item treats this as a
-      // vertical resize, even though none of its ancestors are necessarily
-      // being vertically resized.
-      // (Note: We don't have to do this for width, because InitResizeFlags
-      // will always turn on mHResize on when it sees that the computed width
-      // is different from current width, and that's all we need.)
-      bool forceVerticalResizeForMeasuringReflow =
+      // If this item is flexible (in its block axis)...
+      // OR if we're measuring its 'auto' min-BSize, with its main-size (in its
+      // block axis) being something non-"auto"...
+      // THEN: we assume that the computed BSize that we're reflowing with now
+      // could be different from the one we'll use for this flex item's
+      // "actual" reflow later on.  In that case, we need to be sure the flex
+      // item treats this as a block-axis resize (regardless of whether there
+      // are actually any ancestors being resized in that axis).
+      // (Note: We don't have to do this for the inline axis, because
+      // InitResizeFlags will always turn on mIsIResize on when it sees that
+      // the computed ISize is different from current ISize, and that's all we
+      // need.)
+      bool forceBResizeForMeasuringReflow =
         !aFlexItem.IsFrozen() ||         // Is the item flexible?
         !flexBasisNeedsToMeasureContent; // Are we *only* measuring it for
-                                         // 'min-height:auto'?
+                                         // 'min-block-size:auto'?
 
-      nscoord contentHeight =
-        MeasureFlexItemContentHeight(aPresContext, aFlexItem,
-                                     forceVerticalResizeForMeasuringReflow,
-                                     *flexContainerRI);
+      nscoord contentBSize =
+        MeasureFlexItemContentBSize(aPresContext, aFlexItem,
+                                    forceBResizeForMeasuringReflow,
+                                    *flexContainerRI);
       if (minSizeNeedsToMeasureContent) {
-        resolvedMinSize = std::min(resolvedMinSize, contentHeight);
+        resolvedMinSize = std::min(resolvedMinSize, contentBSize);
       }
       if (flexBasisNeedsToMeasureContent) {
-        aFlexItem.SetFlexBaseSizeAndMainSize(contentHeight);
+        aFlexItem.SetFlexBaseSizeAndMainSize(contentBSize);
       }
     }
   }
@@ -1800,7 +1820,7 @@ nsFlexContainerFrame::
  * intrinsic size is marked as dirty (due to a style or DOM change).
  *
  * In particular the computed height may change between measuring reflows due to
- * how the mIsFlexContainerMeasuringReflow flag affects size computation (see
+ * how the mIsFlexContainerMeasuringBSize flag affects size computation (see
  * bug 1336708).
  *
  * Caching it prevents us from doing exponential reflows in cases of deeply
@@ -1812,27 +1832,27 @@ class nsFlexContainerFrame::CachedMeasuringReflowResult
 {
   // Members that are part of the cache key:
   const LogicalSize mAvailableSize;
-  const nscoord mComputedHeight;
+  const nscoord mComputedBSize;
 
   // Members that are part of the cache value:
-  const nscoord mHeight;
+  const nscoord mBSize;
   const nscoord mAscent;
 
 public:
   CachedMeasuringReflowResult(const ReflowInput& aReflowInput,
                               const ReflowOutput& aDesiredSize)
     : mAvailableSize(aReflowInput.AvailableSize())
-    , mComputedHeight(aReflowInput.ComputedHeight())
-    , mHeight(aDesiredSize.Height())
+    , mComputedBSize(aReflowInput.ComputedBSize())
+    , mBSize(aDesiredSize.BSize(aReflowInput.GetWritingMode()))
     , mAscent(aDesiredSize.BlockStartAscent())
   {}
 
   bool IsValidFor(const ReflowInput& aReflowInput) const {
     return mAvailableSize == aReflowInput.AvailableSize() &&
-      mComputedHeight == aReflowInput.ComputedHeight();
+      mComputedBSize == aReflowInput.ComputedBSize();
   }
 
-  nscoord Height() const { return mHeight; }
+  nscoord BSize() const { return mBSize; }
 
   nscoord Ascent() const { return mAscent; }
 };
@@ -1841,7 +1861,7 @@ NS_DECLARE_FRAME_PROPERTY_DELETABLE(CachedFlexMeasuringReflow,
                                     CachedMeasuringReflowResult);
 
 const CachedMeasuringReflowResult&
-nsFlexContainerFrame::MeasureAscentAndHeightForFlexItem(
+nsFlexContainerFrame::MeasureAscentAndBSizeForFlexItem(
   FlexItem& aItem,
   nsPresContext* aPresContext,
   ReflowInput& aChildReflowInput)
@@ -1892,43 +1912,44 @@ nsFlexContainerFrame::MarkIntrinsicISizesDirty()
 
 nscoord
 nsFlexContainerFrame::
-  MeasureFlexItemContentHeight(nsPresContext* aPresContext,
-                               FlexItem& aFlexItem,
-                               bool aForceVerticalResizeForMeasuringReflow,
-                               const ReflowInput& aParentReflowInput)
+  MeasureFlexItemContentBSize(nsPresContext* aPresContext,
+                              FlexItem& aFlexItem,
+                              bool aForceBResizeForMeasuringReflow,
+                              const ReflowInput& aParentReflowInput)
 {
   // Set up a reflow state for measuring the flex item's auto-height:
   WritingMode wm = aFlexItem.Frame()->GetWritingMode();
   LogicalSize availSize = aParentReflowInput.ComputedSize(wm);
   availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
   ReflowInput
-    childRIForMeasuringHeight(aPresContext, aParentReflowInput,
-                              aFlexItem.Frame(), availSize,
-                              nullptr, ReflowInput::CALLER_WILL_INIT);
-  childRIForMeasuringHeight.mFlags.mIsFlexContainerMeasuringHeight = true;
-  childRIForMeasuringHeight.Init(aPresContext);
+    childRIForMeasuringBSize(aPresContext, aParentReflowInput,
+                             aFlexItem.Frame(), availSize,
+                             nullptr, ReflowInput::CALLER_WILL_INIT);
+  childRIForMeasuringBSize.mFlags.mIsFlexContainerMeasuringBSize = true;
+  childRIForMeasuringBSize.Init(aPresContext);
 
   if (aFlexItem.IsStretched()) {
-    childRIForMeasuringHeight.SetComputedWidth(aFlexItem.GetCrossSize());
-    childRIForMeasuringHeight.SetHResize(true);
+    childRIForMeasuringBSize.SetComputedISize(aFlexItem.GetCrossSize());
+    childRIForMeasuringBSize.SetIResize(true);
   }
 
-  if (aForceVerticalResizeForMeasuringReflow) {
-    childRIForMeasuringHeight.SetVResize(true);
+  if (aForceBResizeForMeasuringReflow) {
+    childRIForMeasuringBSize.SetBResize(true);
   }
 
   const CachedMeasuringReflowResult& reflowResult =
-    MeasureAscentAndHeightForFlexItem(aFlexItem, aPresContext,
-                                      childRIForMeasuringHeight);
+    MeasureAscentAndBSizeForFlexItem(aFlexItem, aPresContext,
+                                     childRIForMeasuringBSize);
 
   aFlexItem.SetAscent(reflowResult.Ascent());
 
-  // Subtract border/padding in vertical axis, to get _just_
-  // the effective computed value of the "height" property.
-  nscoord childDesiredHeight = reflowResult.Height() -
-    childRIForMeasuringHeight.ComputedPhysicalBorderPadding().TopBottom();
 
-  return std::max(0, childDesiredHeight);
+  // Subtract border/padding in block axis, to get _just_
+  // the effective computed value of the BSize property.
+  nscoord childDesiredBSize = reflowResult.BSize() -
+    childRIForMeasuringBSize.ComputedLogicalBorderPadding().BStartEnd(wm);
+
+  return std::max(0, childDesiredBSize);
 }
 
 FlexItem::FlexItem(ReflowInput& aFlexItemReflowInput,
@@ -1952,14 +1973,16 @@ FlexItem::FlexItem(ReflowInput& aFlexItemReflowInput,
     mCrossPosn(0),
     mAscent(0),
     mShareOfWeightSoFar(0.0f),
+    mWM(aFlexItemReflowInput.GetWritingMode()),
     mIsFrozen(false),
     mHadMinViolation(false),
     mHadMaxViolation(false),
     mHadMeasuringReflow(false),
     mIsStretched(false),
     mIsStrut(false),
+    mIsInlineAxisMainAxis(aAxisTracker.IsRowOriented() !=
+                          aAxisTracker.GetWritingMode().IsOrthogonalTo(mWM))
     // mNeedsMinSizeAutoResolution is initialized in CheckForMinSizeAuto()
-    mWM(aFlexItemReflowInput.GetWritingMode())
     // mAlignSelf, see below
 {
   MOZ_ASSERT(mFrame, "expecting a non-null child frame");
@@ -1967,6 +1990,10 @@ FlexItem::FlexItem(ReflowInput& aFlexItemReflowInput,
              "placeholder frames should not be treated as flex items");
   MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
              "out-of-flow frames should not be treated as flex items");
+  MOZ_ASSERT(mIsInlineAxisMainAxis ==
+             nsFlexContainerFrame::IsItemInlineAxisMainAxis(mFrame),
+             "public API should be consistent with internal state (about "
+             "whether flex item's inline axis is flex container's main axis)");
 
   const ReflowInput* containerRS = aFlexItemReflowInput.mParentReflowInput;
   if (IsLegacyBox(containerRS->mFrame)) {
@@ -2047,14 +2074,17 @@ FlexItem::FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize,
     mCrossPosn(0),
     mAscent(0),
     mShareOfWeightSoFar(0.0f),
+    // Struts don't do layout, so its WM doesn't matter at this point. So, we
+    // just share container's WM for simplicity:
+    mWM(aContainerWM),
     mIsFrozen(true),
     mHadMinViolation(false),
     mHadMaxViolation(false),
     mHadMeasuringReflow(false),
     mIsStretched(false),
     mIsStrut(true), // (this is the constructor for making struts, after all)
+    mIsInlineAxisMainAxis(true), // (doesn't matter b/c we're not doing layout)
     mNeedsMinSizeAutoResolution(false),
-    mWM(aContainerWM),
     mAlignSelf(NS_STYLE_ALIGN_FLEX_START)
 {
   MOZ_ASSERT(mFrame, "expecting a non-null child frame");
@@ -2171,15 +2201,28 @@ FlexItem::CanMainSizeInfluenceCrossSize(
     return true;
   }
 
-  if (aAxisTracker.IsCrossAxisHorizontal()) {
-    // If the cross axis is horizontal, then changes to the item's main size
-    // (height) can't influence its cross size (width), if the item is a block
-    // with a horizontal writing-mode.
-    // XXXdholbert This doesn't account for vertical writing-modes, items with
-    // aspect ratios, items that are multicol elements, & items that are
-    // multi-line vertical flex containers. In all of those cases, a change to
-    // the height could influence the width.
-    return false;
+  if (IsInlineAxisCrossAxis()) {
+    // If we get here, this function is really asking: "can changes to this
+    // item's block size have an influence on its inline size"?  For blocks and
+    // tables, the answer is "no".
+    if (mFrame->GetType() == nsGkAtoms::blockFrame ||
+        mFrame->GetType() == nsGkAtoms::tableWrapperFrame) {
+      // XXXdholbert (Maybe use an IsFrameOfType query or something more
+      // general to test this across all frame types? For now, I'm just
+      // optimizing for block and table, since those are common containers that
+      // can contain arbitrarily-large subtrees (and that reliably have ISize
+      // being unaffected by BSize, per CSS2).  So optimizing away needless
+      // relayout is possible & especially valuable for these containers.)
+      return false;
+    }
+    // Other opt-outs can go here, as they're identified as being useful
+    // (particularly for containers where an extra reflow is expensive). But in
+    // general, we have to assume that a flexed BSize *could* influence the
+    // ISize. Some examples where this can definitely happen:
+    // * Intrinsically-sized multicol with fixed-ISize columns, which adds
+    // columns (i.e. grows in inline axis) depending on its block size.
+    // * Intrinsically-sized multi-line column-oriented flex container, which
+    // adds flex lines (i.e. grows in inline axis) depending on its block size.
   }
 
   // Default assumption, if we haven't proven otherwise: the resolved main size
@@ -4083,19 +4126,10 @@ nsFlexContainerFrame::SizeItemInCrossAxis(
   ReflowInput& aChildReflowInput,
   FlexItem& aItem)
 {
-  if (aAxisTracker.IsCrossAxisHorizontal()) {
-    MOZ_ASSERT(aItem.HasIntrinsicRatio(),
-               "For now, caller's CanMainSizeInfluenceCrossSize check should "
-               "only allow us to get here for items with intrinsic ratio");
-    // XXXdholbert When we finish support for vertical writing-modes,
-    // (in bug 1079155 or a dependency), we'll relax the horizontal check in
-    // CanMainSizeInfluenceCrossSize, and this function will need to be able
-    // to measure the baseline & width (given our resolved height)
-    // of vertical-writing-mode flex items here.
-    // For now, we only expect to get here for items with an intrinsic aspect
-    // ratio; and for those items, we can just read the size off of the reflow
-    // state, without performing reflow.
-    aItem.SetCrossSize(aChildReflowInput.ComputedWidth());
+  // If cross axis is the item's inline axis, just use ISize from reflow state,
+  // and don't bother with a full reflow.
+  if (aItem.IsInlineAxisCrossAxis()) {
+    aItem.SetCrossSize(aChildReflowInput.ComputedISize());
     return;
   }
 
@@ -4104,16 +4138,18 @@ nsFlexContainerFrame::SizeItemInCrossAxis(
 
   if (aItem.GetAlignSelf() == NS_STYLE_ALIGN_STRETCH) {
     // This item's got "align-self: stretch", so we probably imposed a
-    // stretched computed height on it during its previous reflow. We're
-    // not imposing that height for *this* measuring reflow, so we need to
-    // tell it to treat this reflow as a vertical resize (regardless of
-    // whether any of its ancestors are being resized).
-    aChildReflowInput.SetVResize(true);
+    // stretched computed cross-size on it during its previous
+    // reflow. We're not imposing that BSize for *this* "measuring" reflow, so
+    // we need to tell it to treat this reflow as a resize in its block axis
+    // (regardless of whether any of its ancestors are actually being resized).
+    // (Note: we know that the cross axis is the item's *block* axis -- if it
+    // weren't, then we would've taken the early-return above.)
+    aChildReflowInput.SetBResize(true);
   }
 
   // Potentially reflow the item, and get the sizing info.
   const CachedMeasuringReflowResult& reflowResult =
-    MeasureAscentAndHeightForFlexItem(aItem, aPresContext, aChildReflowInput);
+    MeasureAscentAndBSizeForFlexItem(aItem, aPresContext, aChildReflowInput);
 
   // Save the sizing info that we learned from this reflow
   // -----------------------------------------------------
@@ -4125,7 +4161,7 @@ nsFlexContainerFrame::SizeItemInCrossAxis(
   // so we don't bother with making aAxisTracker pick the cross-axis component
   // for us.)
   nscoord crossAxisBorderPadding = aItem.GetBorderPadding().TopBottom();
-  if (reflowResult.Height() < crossAxisBorderPadding) {
+  if (reflowResult.BSize() < crossAxisBorderPadding) {
     // Child's requested size isn't large enough for its border/padding!
     // This is OK for the trivial nsFrame::Reflow() impl, but other frame
     // classes should know better. So, if we get here, the child had better be
@@ -4138,7 +4174,7 @@ nsFlexContainerFrame::SizeItemInCrossAxis(
     aItem.SetCrossSize(0);
   } else {
     // (normal case)
-    aItem.SetCrossSize(reflowResult.Height() - crossAxisBorderPadding);
+    aItem.SetCrossSize(reflowResult.BSize() - crossAxisBorderPadding);
   }
 
   aItem.SetAscent(reflowResult.Ascent());
@@ -4389,6 +4425,41 @@ nsFlexContainerFrame::CalculatePackingSpace(uint32_t aNumThingsToPack,
   *aPackingSpaceRemaining -= totalEdgePackingSpace;
 }
 
+/* static */
+bool
+nsFlexContainerFrame::IsItemInlineAxisMainAxis(nsIFrame* aFrame)
+{
+  MOZ_ASSERT(aFrame && aFrame->IsFlexItem(), "expecting arg to be a flex item");
+  const WritingMode flexItemWM = aFrame->GetWritingMode();
+  const nsIFrame* flexContainer = aFrame->GetParent();
+
+  if (IsLegacyBox(flexContainer)) {
+    // For legacy boxes, the main axis is determined by "box-orient", and we can
+    // just directly check if that's vertical, and compare that to whether the
+    // item's WM is also vertical:
+    bool boxOrientIsVertical =
+      (flexContainer->StyleXUL()->mBoxOrient == StyleBoxOrient::Vertical);
+    return flexItemWM.IsVertical() == boxOrientIsVertical;
+  }
+
+  // For modern CSS flexbox, we get our return value by asking two questions
+  // and comparing their answers.
+  // Question 1: does aFrame have the same inline axis as its flex container?
+  bool itemInlineAxisIsParallelToParent =
+    !flexItemWM.IsOrthogonalTo(flexContainer->GetWritingMode());
+
+  // Question 2: is aFrame's flex container row-oriented? (This tells us
+  // whether the flex container's main axis is its inline axis.)
+  auto flexDirection = flexContainer->StylePosition()->mFlexDirection;
+  bool flexContainerIsRowOriented =
+    flexDirection == NS_STYLE_FLEX_DIRECTION_ROW ||
+    flexDirection == NS_STYLE_FLEX_DIRECTION_ROW_REVERSE;
+
+  // aFrame's inline axis is its flex container's main axis IFF the above
+  // questions have the same answer.
+  return flexContainerIsRowOriented == itemInlineAxisIsParallelToParent;
+}
+
 void
 nsFlexContainerFrame::DoFlexLayout(nsPresContext*           aPresContext,
                                    ReflowOutput&     aDesiredSize,
diff --git a/layout/generic/nsFlexContainerFrame.h b/layout/generic/nsFlexContainerFrame.h
index 39de79b577..e349709876 100644
--- a/layout/generic/nsFlexContainerFrame.h
+++ b/layout/generic/nsFlexContainerFrame.h
@@ -133,6 +133,21 @@ public:
                                     uint32_t* aNumPackingSpacesRemaining,
                                     nscoord* aPackingSpaceRemaining);
 
+  /**
+   * Given a frame for a flex item, this method returns true IFF that flex
+   * item's inline axis is the same as (i.e. not orthogonal to) its flex
+   * container's main axis.
+   *
+   * (This method is only intended to be used from external
+   * callers. Inside of flex reflow code, FlexItem::IsInlineAxisMainAxis() is
+   * equivalent & more optimal.)
+   *
+   * @param aFrame a flex item (must return true from IsFlexItem)
+   * @return true iff aFrame's inline axis is the same as (i.e. not orthogonal
+   *              to) its flex container's main axis. Otherwise, false.
+   */
+  static bool IsItemInlineAxisMainAxis(nsIFrame* aFrame);
+
 protected:
   // Protected constructor & destructor
   explicit nsFlexContainerFrame(nsStyleContext* aContext)
@@ -207,20 +222,20 @@ protected:
    * This avoids exponential reflows, see the comment on
    * CachedMeasuringReflowResult.
    */
-  const CachedMeasuringReflowResult& MeasureAscentAndHeightForFlexItem(
+  const CachedMeasuringReflowResult& MeasureAscentAndBSizeForFlexItem(
     FlexItem& aItem,
     nsPresContext* aPresContext,
     ReflowInput& aChildReflowInput);
 
   /**
-   * This method performs a "measuring" reflow to get the content height of
-   * aFlexItem.Frame() (treating it as if it had auto-height), & returns the
-   * resulting height.
+   * This method performs a "measuring" reflow to get the content BSize of
+   * aFlexItem.Frame() (treating it as if it had a computed BSize of "auto"),
+   * and returns the resulting BSize measurement.
    * (Helper for ResolveAutoFlexBasisAndMinSize().)
    */
-  nscoord MeasureFlexItemContentHeight(nsPresContext* aPresContext,
+  nscoord MeasureFlexItemContentBSize(nsPresContext* aPresContext,
                                        FlexItem& aFlexItem,
-                                       bool aForceVerticalResizeForMeasuringReflow,
+                                       bool aForceBSizeForMeasuringReflow,
                                        const ReflowInput& aParentReflowInput);
 
   /**
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 43ad970890..9bc4941e0b 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -39,6 +39,7 @@
 #include "mozilla/Logging.h"
 #include "mozilla/Sprintf.h"
 #include "nsFrameManager.h"
+#include "nsFlexContainerFrame.h"
 #include "nsLayoutUtils.h"
 #include "LayoutLogging.h"
 #include "mozilla/RestyleManager.h"
@@ -4752,12 +4753,11 @@ nsFrame::ComputeSize(nsRenderingContext* aRenderingContext,
   }
   bool isFlexItem = (parentFrameType == nsGkAtoms::flexContainerFrame &&
                      !(GetStateBits() & NS_FRAME_OUT_OF_FLOW));
+
   bool isInlineFlexItem = false;
   if (isFlexItem) {
-    uint32_t flexDirection = GetParent()->StylePosition()->mFlexDirection;
     isInlineFlexItem =
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW ||
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW_REVERSE;
+      nsFlexContainerFrame::IsItemInlineAxisMainAxis(this);
 
     const nsStyleCoord* flexBasis = &(stylePos->mFlexBasis);
     SetCoordToFlexBasis(isInlineFlexItem, false, flexBasis,
@@ -4973,11 +4973,8 @@ nsFrame::ComputeSizeWithIntrinsicDimensions(nsRenderingContext*  aRenderingConte
   // from our style struct. (Otherwise, we'll be using an irrelevant value in
   // the aspect-ratio calculations below.)
   if (isFlexItem) {
-    uint32_t flexDirection =
-      GetParent()->StylePosition()->mFlexDirection;
     isInlineFlexItem =
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW ||
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW_REVERSE;
+      nsFlexContainerFrame::IsItemInlineAxisMainAxis(this);
 
     // If FlexItemMainSizeOverride frame-property is set, then that means the
     // flex container is imposing a main-size on this flex item for it to use
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-block-horiz-001v.xhtml b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-block-horiz-001v.xhtml
new file mode 100644
index 0000000000..f1dfdfdb86
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-block-horiz-001v.xhtml
@@ -0,0 +1,73 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!-- Testcase with blocks as flex items in a horizontal flex container, with
+     various "flex" values and various combinations of the items
+     and with various writing-modes on the items. -->
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>
+      CSS Test: Testing flexbox layout algorithm property on block flex items in a horizontal flex container
+      (with various writing-modes on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com"/>
+    <link rel="help" href="http://www.w3.org/TR/css-flexbox-1/#layout-algorithm"/>
+    <link rel="match" href="flexbox-basic-block-horiz-001-ref.xhtml"/>
+    <style>
+      div { height: 100px; }
+      div.flexbox {
+        border: 1px dashed blue;
+        width: 200px;
+        font-size: 10px;
+        display: flex;
+      }
+      div.a {
+        flex: 1 0 30px;
+        background: lightgreen;
+        writing-mode: vertical-lr;
+      }
+      div.b {
+        flex: 2 0 20px;
+        background: yellow;
+        writing-mode: vertical-rl;
+      }
+      div.c {
+        flex: 3 0 40px;
+        background: orange;
+        writing-mode: sideways-lr;
+      }
+      div.flexNone {
+        flex: none;
+        background: pink;
+        writing-mode: vertical-lr;
+      }
+      div.flexBasis {
+        flex: 0 0 20px;
+        background: gray;
+        writing-mode: sideways-rl;
+      }
+      div.spacer {
+        width: 15px;
+        height: 15px;
+        background: purple;
+      }
+    </style>
+  </head>
+  <body>
+    <div class="flexbox"><div class="a"></div><div class="b"/></div>
+    <div class="flexbox"><div class="a"/><div class="c"/></div>
+    <div class="flexbox"><div class="a"/>
+      <div class="flexNone"><div class="spacer"/></div>
+    </div>
+    <div class="flexbox"><div class="b"/><div class="c"/></div>
+    <div class="flexbox"><div class="b"/>
+      <div class="flexNone"><div class="spacer"/><div class="spacer"/></div>
+    </div>
+
+    <div class="flexbox">
+      <div class="a"/><div class="b"/><div class="flexBasis"/><div class="c"/>
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-block-vert-001v.xhtml b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-block-vert-001v.xhtml
new file mode 100644
index 0000000000..da943dfc56
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-block-vert-001v.xhtml
@@ -0,0 +1,75 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!-- Testcase with blocks as flex items in a vertical flex container, with
+     various "flex" values and various combinations of the items. -->
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>
+      CSS Test: Testing flexbox layout algorithm property on block flex items in a vertical flex container
+      (with various writing-modes on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com"/>
+    <link rel="help" href="http://www.w3.org/TR/css-flexbox-1/#layout-algorithm"/>
+    <link rel="match" href="flexbox-basic-block-vert-001-ref.xhtml"/>
+    <style>
+      div { width: 50px; }
+      div.flexbox {
+        float: left;
+        border: 1px dashed blue;
+        height: 200px;
+        font-size: 10px;
+        display: flex;
+        flex-direction: column;
+      }
+      div.a {
+        flex: 1 0 30px;
+        background: lightgreen;
+        writing-mode: vertical-lr;
+      }
+      div.b {
+        flex: 2 0 20px;
+        background: yellow;
+        writing-mode: vertical-rl;
+      }
+      div.c {
+        flex: 3 0 40px;
+        background: orange;
+        writing-mode: sideways-lr;
+      }
+      div.flexNone {
+        flex: none;
+        background: pink;
+        writing-mode: vertical-lr;
+      }
+      div.flexBasis {
+        flex: 0 0 20px;
+        background: gray;
+        writing-mode: sideways-rl;
+      }
+      div.spacer {
+        display: inline-block;
+        width: 15px;
+        height: 15px;
+        background: purple;
+      }
+    </style>
+  </head>
+  <body>
+    <div class="flexbox"><div class="a"></div><div class="b"/></div>
+    <div class="flexbox"><div class="a"/><div class="c"/></div>
+    <div class="flexbox"><div class="a"/>
+      <div class="flexNone"><div class="spacer"/></div>
+    </div>
+    <div class="flexbox"><div class="b"/><div class="c"/></div>
+    <div class="flexbox"><div class="b"/>
+      <div class="flexNone"><div class="spacer"/><div class="spacer"/></div>
+    </div>
+
+    <div class="flexbox">
+      <div class="a"/><div class="b"/><div class="flexBasis"/><div class="c"/>
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-canvas-horiz-001v.xhtml b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-canvas-horiz-001v.xhtml
new file mode 100644
index 0000000000..ad623d77f7
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-canvas-horiz-001v.xhtml
@@ -0,0 +1,105 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!--
+     This test checks that canvas elements behave correctly as flex items.
+-->
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>
+      CSS Test: Testing flexbox layout algorithm property on canvas flex items in a horizontal flex container
+      (with a vertical writing-mode on the canvas flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com"/>
+    <link rel="help" href="http://www.w3.org/TR/css-flexbox-1/#layout-algorithm"/>
+    <link rel="match" href="flexbox-basic-canvas-horiz-001-ref.xhtml"/>
+    <style>
+      div.flexbox {
+        width: 200px;
+        background: lightgreen;
+        display: flex;
+        justify-content: space-between;
+        margin-bottom: 5px;
+        line-height: 8px;
+      }
+      canvas {
+        min-width: 0;
+        width: 10px;
+        height: 20px;
+        border: 1px dotted green;
+        writing-mode: vertical-lr;
+      }
+    </style>
+  </head>
+  <body>
+
+    <!-- A) One flex item -->
+    <div class="flexbox">
+      <canvas/>
+    </div>
+
+    <!-- B) Text and a canvas (ensure they aren't merged into one flex item) -->
+    <div class="flexbox">
+      some words <canvas/>
+    </div>
+
+    <!-- C) Two canvas elements, getting stretched by different ratios, from 0.
+
+         Space-to-be-distributed = 200px - borders = 200 - (1 + 1) - (1 + 1)
+                                 = 196px
+
+         1st element gets 5/8 of space: 5/8 * 196px = 122.5px
+         1st element gets 3/8 of space: 3/8 * 196px = 73.5px
+      -->
+    <div class="flexbox">
+      <canvas style="flex: 5"/>
+      <canvas style="flex: 3"/>
+    </div>
+
+    <!-- D) Two canvas elements, getting stretched by different ratios, from
+         different flex bases.
+
+         Space-to-be-distributed = 200px - (33 + 1 + 1) - (13 + 1 + 1) = 150px
+         1st element gets 2/5 of space: 33px + 2/5 * 150px = 93px
+         1st element gets 3/5 of space: 13px + 3/5 * 150px = 103px
+      -->
+    <div class="flexbox">
+      <canvas style="width: 33px; flex: 2 auto"/>
+      <canvas style="width: 13px; flex: 3 auto"/>
+    </div>
+
+    <!-- E) Two flex items, getting shrunk by different amounts.
+
+         Space-to-be-distributed = 200px - (150 + 1 + 1) - (100 + 1 + 1) = -54px
+         First element scaled flex ratio = 4 * 150 = 600
+         Second element scaled flex ratio = 3 * 100 = 300
+           * So, total flexibility is 600 + 300 = 900
+
+         1st element gets 600/900 of space: 150 + 600/900 * -54 = 114px
+         2nd element gets 300/900 of space: 100 + 300/900 * -54 = 82px
+      -->
+    <div class="flexbox">
+      <canvas style="width: 150px; flex: 1 4 auto"/>
+      <canvas style="width: 100px; flex: 1 3 auto"/>
+    </div>
+
+    <!-- F) Making sure we don't grow past max-width -->
+    <!-- Same as (D), except we've added a max-width on the second element.
+      -->
+    <div class="flexbox">
+      <canvas style="width: 33px; flex: 2 auto"/>
+      <canvas style="width: 13px; max-width: 90px; flex: 3 auto"/>
+    </div>
+
+    <!-- G) Making sure we grow at least as large as min-width -->
+    <!-- Same as (C), except we've added a min-width on the second element.
+      -->
+    <div class="flexbox">
+      <canvas style="width: 33px; flex: 2 auto"/>
+      <canvas style="width: 13px; min-width: 150px; flex: 3 auto"/>
+    </div>
+
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-canvas-vert-001v.xhtml b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-canvas-vert-001v.xhtml
new file mode 100644
index 0000000000..057f63ae62
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-basic-canvas-vert-001v.xhtml
@@ -0,0 +1,107 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!--
+     This test checks that canvas elements behave correctly as flex items.
+-->
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>
+      CSS Test: Testing flexbox layout algorithm property on canvas flex items in a vertical flex container
+      (with a vertical writing-mode on the canvas flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com"/>
+    <link rel="help" href="http://www.w3.org/TR/css-flexbox-1/#layout-algorithm"/>
+    <link rel="match" href="flexbox-basic-canvas-vert-001-ref.xhtml"/>
+    <style>
+      div.flexbox {
+        height: 200px;
+        background: lightgreen;
+        display: flex;
+        justify-content: space-between;
+        flex-direction: column;
+        float: left;
+        margin-right: 10px;
+        font: 8px monospace;
+      }
+      canvas {
+        width: 20px;
+        height: 10px;
+        min-height: 0;
+        border: 1px dotted green;
+        writing-mode: vertical-lr;
+      }
+    </style>
+  </head>
+  <body>
+
+    <!-- A) One flex item -->
+    <div class="flexbox">
+      <canvas/>
+    </div>
+
+    <!-- B) Text and a canvas (ensure they aren't merged into one flex item) -->
+    <div class="flexbox">
+      a b <canvas/>
+    </div>
+
+    <!-- C) Two canvas elements, getting stretched by different ratios, from 0.
+
+         Space-to-be-distributed = 200px - borders = 200 - (1 + 1) - (1 + 1)
+                                 = 196px
+
+         1st element gets 5/8 of space: 5/8 * 196px = 122.5px
+         1st element gets 3/8 of space: 3/8 * 196px = 73.5px
+      -->
+    <div class="flexbox">
+      <canvas style="flex: 5"/>
+      <canvas style="flex: 3"/>
+    </div>
+
+    <!-- D) Two canvas elements, getting stretched by different ratios, from
+         different flex bases.
+
+         Space-to-be-distributed = 200px - (33 + 1 + 1) - (13 + 1 + 1) = 150px
+         1st element gets 2/5 of space: 33px + 2/5 * 150px = 93px
+         1st element gets 3/5 of space: 13px + 3/5 * 150px = 103px
+      -->
+    <div class="flexbox">
+      <canvas style="height: 33px; flex: 2 auto"/>
+      <canvas style="height: 13px; flex: 3 auto"/>
+    </div>
+
+    <!-- E) Two flex items, getting shrunk by different amounts.
+
+         Space-to-be-distributed = 200px - (150 + 1 + 1) - (100 + 1 + 1) = -54px
+         First element scaled flex ratio = 4 * 150 = 600
+         Second element scaled flex ratio = 3 * 100 = 300
+           * So, total flexibility is 600 + 300 = 900
+
+         1st element gets 600/900 of space: 150 + 600/900 * -54 = 114px
+         2nd element gets 300/900 of space: 100 + 300/900 * -54 = 82px
+      -->
+    <div class="flexbox">
+      <canvas style="height: 150px; flex: 1 4 auto"/>
+      <canvas style="height: 100px; flex: 1 3 auto"/>
+    </div>
+
+    <!-- F) Making sure we don't grow past max-height -->
+    <!-- Same as (D), except we've added a max-height on the second element.
+      -->
+    <div class="flexbox">
+      <canvas style="height: 33px; flex: 2 auto"/>
+      <canvas style="height: 13px; max-height: 90px; flex: 3 auto"/>
+    </div>
+
+    <!-- G) Making sure we grow at least as large as min-height -->
+    <!-- Same as (C), except we've added a min-height on the second element.
+      -->
+    <div class="flexbox">
+      <canvas style="height: 33px; flex: 2 auto"/>
+      <canvas style="height: 13px; min-height: 150px; flex: 3 auto"/>
+    </div>
+
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-001v.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-001v.html
new file mode 100644
index 0000000000..4cc910b9e2
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-001v.html
@@ -0,0 +1,128 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>
+      CSS Test: Testing how explicit main-size & cross-size constraints
+      influence sizing on non-stretched flex item w/ intrinsic ratio
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-main-size">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-cross-size">
+    <link rel="match" href="flexbox-intrinsic-ratio-001-ref.html">
+    <style>
+      .flexbox {
+        display: flex;
+        flex-direction: row;
+        border: 1px solid black;
+        margin: 0 2px 2px 0; /* (Just for spacing things out, visually) */
+        width: 40px;
+        height: 40px;
+
+        justify-content: flex-start;
+        align-items: flex-start;
+
+        float: left; /* For testing in "rows" */
+      }
+      br { clear: both; }
+
+      .flexbox > * {
+        writing-mode: vertical-lr;
+
+        /* Disable "min-width:auto"/"min-height:auto" to focus purely on
+           later channels of influence. */
+        min-width: 0;
+        min-height: 0;
+      }
+    </style>
+  </head>
+  <body>
+    <!-- NOTE: solidblue.png has an intrinsic size of 16px by 16px. -->
+
+    <!-- Row 1: no special sizing: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png">
+    </div>
+    <br>
+
+    <!-- Row 2: Specified main-size, cross-size, or flex-basis: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="flex: 0 0 30px">
+    </div>
+    <br>
+
+    <!-- Row 3: min main-size OR min cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 4: max main-size OR max cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              max-height: 6px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 6px;
+                                              max-height: 10px">
+    </div>
+    <br>
+
+    <!-- Row 5: min main-size vs. max cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 6: min|max main-size vs. explicit cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 10px;
+                                              min-height: 30px">
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-002v.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-002v.html
new file mode 100644
index 0000000000..0df92f9954
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-002v.html
@@ -0,0 +1,128 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>
+      CSS Test: Testing how explicit main-size & cross-size constraints
+      influence sizing on non-stretched flex item w/ intrinsic ratio
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-main-size">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-cross-size">
+    <link rel="match" href="flexbox-intrinsic-ratio-001-ref.html">
+    <style>
+      .flexbox {
+        display: flex;
+        flex-direction: column;
+        border: 1px solid black;
+        margin: 0 2px 2px 0; /* (Just for spacing things out, visually) */
+        width: 40px;
+        height: 40px;
+
+        justify-content: flex-start;
+        align-items: flex-start;
+
+        float: left; /* For testing in "rows" */
+      }
+      br { clear: both; }
+
+      .flexbox > * {
+        writing-mode: vertical-lr;
+
+        /* Disable "min-width:auto"/"min-height:auto" to focus purely on
+           later channels of influence. */
+        min-width: 0;
+        min-height: 0;
+      }
+    </style>
+  </head>
+  <body>
+    <!-- NOTE: solidblue.png has an intrinsic size of 16px by 16px. -->
+
+    <!-- Row 1: no special sizing: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png">
+    </div>
+    <br>
+
+    <!-- Row 2: Specified main-size, cross-size, or flex-basis: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="flex: 0 0 30px">
+    </div>
+    <br>
+
+    <!-- Row 3: min main-size OR min cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 4: max main-size OR max cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              max-height: 6px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 6px;
+                                              max-height: 10px">
+    </div>
+    <br>
+
+    <!-- Row 5: min main-size vs. max cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 6: min|max main-size vs. explicit cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 10px;
+                                              min-height: 30px">
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-003v.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-003v.html
new file mode 100644
index 0000000000..cb9275fd8c
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-003v.html
@@ -0,0 +1,128 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>
+      CSS Test: Testing how explicit main-size & cross-size constraints
+      influence sizing on stretched flex item w/ intrinsic ratio
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-main-size">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-cross-size">
+    <link rel="match" href="flexbox-intrinsic-ratio-003-ref.html">
+    <style>
+      .flexbox {
+        display: flex;
+        flex-direction: row;
+        border: 1px solid black;
+        margin: 0 2px 2px 0; /* (Just for spacing things out, visually) */
+        width: 40px;
+        height: 40px;
+
+        justify-content: flex-start;
+        align-items: stretch;
+
+        float: left; /* For testing in "rows" */
+      }
+      br { clear: both; }
+
+      .flexbox > * {
+        writing-mode: vertical-rl;
+
+        /* Disable "min-width:auto"/"min-height:auto" to focus purely on
+           later channels of influence. */
+        min-width: 0;
+        min-height: 0;
+      }
+    </style>
+  </head>
+  <body>
+    <!-- NOTE: solidblue.png has an intrinsic size of 16px by 16px. -->
+
+    <!-- Row 1: no special sizing: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png">
+    </div>
+    <br>
+
+    <!-- Row 2: Specified main-size, cross-size, or flex-basis: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="flex: 0 0 30px">
+    </div>
+    <br>
+
+    <!-- Row 3: min main-size OR min cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 4: max main-size OR max cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              max-height: 6px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 6px;
+                                              max-height: 10px">
+    </div>
+    <br>
+
+    <!-- Row 5: min main-size vs. max cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 6: min|max main-size vs. explicit cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 10px;
+                                              min-height: 30px">
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-004v.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-004v.html
new file mode 100644
index 0000000000..01c5271c45
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-004v.html
@@ -0,0 +1,128 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>
+      CSS Test: Testing how explicit main-size & cross-size constraints
+      influence sizing on stretched flex item w/ intrinsic ratio
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-main-size">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-cross-size">
+    <link rel="match" href="flexbox-intrinsic-ratio-004-ref.html">
+    <style>
+      .flexbox {
+        display: flex;
+        flex-direction: column;
+        border: 1px solid black;
+        margin: 0 2px 2px 0; /* (Just for spacing things out, visually) */
+        width: 40px;
+        height: 40px;
+
+        justify-content: flex-start;
+        align-items: stretch;
+
+        float: left; /* For testing in "rows" */
+      }
+      br { clear: both; }
+
+      .flexbox > * {
+        writing-mode: vertical-rl;
+
+        /* Disable "min-width:auto"/"min-height:auto" to focus purely on
+           later channels of influence. */
+        min-width: 0;
+        min-height: 0;
+      }
+    </style>
+  </head>
+  <body>
+    <!-- NOTE: solidblue.png has an intrinsic size of 16px by 16px. -->
+
+    <!-- Row 1: no special sizing: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png">
+    </div>
+    <br>
+
+    <!-- Row 2: Specified main-size, cross-size, or flex-basis: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="flex: 0 0 30px">
+    </div>
+    <br>
+
+    <!-- Row 3: min main-size OR min cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 4: max main-size OR max cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              max-height: 6px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 6px;
+                                              max-height: 10px">
+    </div>
+    <br>
+
+    <!-- Row 5: min main-size vs. max cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 6: min|max main-size vs. explicit cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 10px;
+                                              min-height: 30px">
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-005v.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-005v.html
new file mode 100644
index 0000000000..ed1dcace0f
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-005v.html
@@ -0,0 +1,129 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>
+      CSS Test: Testing how explicit main-size & cross-size constraints
+      influence sizing on non-stretched flexible flex item w/ intrinsic ratio
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-main-size">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-cross-size">
+    <link rel="match" href="flexbox-intrinsic-ratio-005-ref.html">
+    <style>
+      .flexbox {
+        display: flex;
+        flex-direction: row;
+        border: 1px solid black;
+        margin: 0 2px 2px 0; /* (Just for spacing things out, visually) */
+        width: 40px;
+        height: 40px;
+
+        justify-content: flex-start;
+        align-items: flex-start;
+
+        float: left; /* For testing in "rows" */
+      }
+      br { clear: both; }
+
+      .flexbox > * {
+        flex: 1;
+        writing-mode: vertical-lr;
+
+        /* Disable "min-width:auto"/"min-height:auto" to focus purely on
+           later channels of influence. */
+        min-width: 0;
+        min-height: 0;
+      }
+    </style>
+  </head>
+  <body>
+    <!-- NOTE: solidblue.png has an intrinsic size of 16px by 16px. -->
+
+    <!-- Row 1: no special sizing: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png">
+    </div>
+    <br>
+
+    <!-- Row 2: Specified main-size, cross-size, or flex-basis: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="flex: 1 1 30px">
+    </div>
+    <br>
+
+    <!-- Row 3: min main-size OR min cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 4: max main-size OR max cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              max-height: 6px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 6px;
+                                              max-height: 10px">
+    </div>
+    <br>
+
+    <!-- Row 5: min main-size vs. max cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 6: min|max main-size vs. explicit cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 10px;
+                                              min-height: 30px">
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-006v.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-006v.html
new file mode 100644
index 0000000000..8c12fad8c7
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-intrinsic-ratio-006v.html
@@ -0,0 +1,129 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>
+      CSS Test: Testing how explicit main-size & cross-size constraints
+      influence sizing on non-stretched flexible flex item w/ intrinsic ratio
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-main-size">
+    <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#hypothetical-cross-size">
+    <link rel="match" href="flexbox-intrinsic-ratio-006-ref.html">
+    <style>
+      .flexbox {
+        display: flex;
+        flex-direction: column;
+        border: 1px solid black;
+        margin: 0 2px 2px 0; /* (Just for spacing things out, visually) */
+        width: 40px;
+        height: 40px;
+
+        justify-content: flex-start;
+        align-items: flex-start;
+
+        float: left; /* For testing in "rows" */
+      }
+      br { clear: both; }
+
+      .flexbox > * {
+        flex: 1;
+        writing-mode: vertical-lr;
+
+        /* Disable "min-width:auto"/"min-height:auto" to focus purely on
+           later channels of influence. */
+        min-width: 0;
+        min-height: 0;
+      }
+    </style>
+  </head>
+  <body>
+    <!-- NOTE: solidblue.png has an intrinsic size of 16px by 16px. -->
+
+    <!-- Row 1: no special sizing: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png">
+    </div>
+    <br>
+
+    <!-- Row 2: Specified main-size, cross-size, or flex-basis: -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="flex: 1 1 30px">
+    </div>
+    <br>
+
+    <!-- Row 3: min main-size OR min cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              min-height: 34px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 34px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 4: max main-size OR max cross-size, or both -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              max-height: 6px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 6px;
+                                              max-height: 10px">
+    </div>
+    <br>
+
+    <!-- Row 5: min main-size vs. max cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              min-height: 30px">
+    </div>
+    <br>
+
+    <!-- Row 6: min|max main-size vs. explicit cross-size, & vice versa -->
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="min-width: 30px;
+                                              height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 30px;
+                                              max-height: 10px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="max-width: 10px;
+                                              height: 30px">
+    </div>
+    <div class="flexbox">
+      <img src="support/solidblue.png" style="width: 10px;
+                                              min-height: 30px">
+    </div>
+  </body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-mbp-horiz-002v.xhtml b/layout/reftests/w3c-css/submitted/flexbox/flexbox-mbp-horiz-002v.xhtml
new file mode 100644
index 0000000000..a02e3406c9
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-mbp-horiz-002v.xhtml
@@ -0,0 +1,86 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!-- Testcase with margin/border/padding on flex items
+     and with various writing-modes on the items. -->
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>
+      CSS Test: Testing margins, borders, and padding on flex items in a horizontal flex container
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com"/>
+    <link rel="help" href="http://www.w3.org/TR/css-flexbox-1/#layout-algorithm"/>
+    <link rel="match" href="flexbox-mbp-horiz-002-ref.xhtml"/>
+    <style>
+      div { height: 100px; border: 0; }
+      div.flexbox {
+        width: 200px;
+        font-size: 10px;
+        display: flex;
+      }
+      div.a {
+        flex: 1 0 9px;
+        background: lightgreen;
+        margin-left: 1px;
+        margin-right: 3px;
+        border-style: dotted;
+        border-left-width: 2px;
+        border-right-width: 4px;
+        padding-left: 5px;
+        padding-right: 6px;
+        writing-mode: vertical-lr;
+      }
+      div.b {
+        flex: 2 0 1px;
+        background: yellow;
+        margin-left: 2px;
+        margin-right: 4px;
+        border-style: dashed;
+        border-left-width: 7px;
+        border-right-width: 3px;
+        padding-left: 1px;
+        padding-right: 2px;
+        writing-mode: vertical-rl;
+      }
+      div.c {
+        flex: 3 0 40px;
+        background: orange;
+        writing-mode: sideways-lr;
+      }
+      div.flexNone {
+        flex: none;
+        background: pink;
+        writing-mode: vertical-lr;
+      }
+      div.flexBasis {
+        flex: 0 0 20px;
+        background: gray;
+        writing-mode: sideways-rl;
+      }
+      div.spacer {
+        width: 15px;
+        height: 15px;
+        background: purple;
+      }
+    </style>
+  </head>
+  <body>
+    <div class="flexbox"><div class="a"></div><div class="b"/></div>
+    <div class="flexbox"><div class="a"/><div class="c"/></div>
+    <div class="flexbox"><div class="a"/>
+      <div class="flexNone"><div class="spacer"/></div>
+    </div>
+    <div class="flexbox"><div class="b"/><div class="c"/></div>
+    <div class="flexbox"><div class="b"/>
+      <div class="flexNone"><div class="spacer"/><div class="spacer"/></div>
+    </div>
+
+    <div class="flexbox">
+      <div class="a"/><div class="b"/><div class="flexBasis"/><div class="c"/>
+    </div>
+  </body>
+</html>
+
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-mbp-horiz-003v.xhtml b/layout/reftests/w3c-css/submitted/flexbox/flexbox-mbp-horiz-003v.xhtml
new file mode 100644
index 0000000000..8baef30227
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-mbp-horiz-003v.xhtml
@@ -0,0 +1,88 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!-- Testcase with border/padding on a flex container and on its children -->
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>
+      CSS Test: Testing borders and padding on a horizontal flex container and its flex items
+      (with a vertical writing-mode on the flex items).
+    </title>
+    <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com"/>
+    <link rel="help" href="http://www.w3.org/TR/css-flexbox-1/#layout-algorithm"/>
+    <link rel="match" href="flexbox-mbp-horiz-003-ref.xhtml"/>
+    <style>
+      div { height: 20px; border: 0; }
+      div.flexbox {
+        width: 200px;
+        display: flex;
+        margin-bottom: 2px;
+      }
+
+      <!-- customizations for flex container border/padding -->
+      .borderA {
+        border-style: dashed;
+        border-color: purple;
+        border-top-width: 6px;
+        border-right-width: 4px;
+        border-bottom-width: 2px;
+        border-left-width: 8px;
+      }
+
+      .borderB {
+        border-style: dashed;
+        border-color: purple;
+        border-top-width: 4px;
+        border-right-width: 5px;
+        border-bottom-width: 6px;
+        border-left-width: 7px;
+      }
+
+      .paddingA {
+        padding: 4px 3px 2px 1px;
+      }
+
+      .paddingB {
+        padding: 8px 11px 14px 17px;
+      }
+
+      div.child1 {
+        flex: 1 0 24px;
+        background: lightgreen;
+        border-style: dotted;
+        border-left-width: 2px;
+        border-right-width: 4px;
+        writing-mode: vertical-rl;
+      }
+      div.child2 {
+        flex: 2 0 10px;
+        background: yellow;
+        border-style: dashed;
+        border-left-width: 7px;
+        border-right-width: 3px;
+        writing-mode: vertical-lr;
+      }
+    </style>
+  </head>
+  <body>
+    <div class="flexbox borderA"
+         ><div class="child1"/><div class="child2"/></div>
+    <div class="flexbox borderA paddingA"
+         ><div class="child1"/><div class="child2"/></div>
+    <div class="flexbox borderA paddingB"
+         ><div class="child1"/><div class="child2"/></div>
+    <div class="flexbox borderB"
+         ><div class="child1"/><div class="child2"/></div>
+    <div class="flexbox borderB paddingA"
+         ><div class="child1"/><div class="child2"/></div>
+    <div class="flexbox borderB paddingB"
+         ><div class="child1"/><div class="child2"/></div>
+    <div class="flexbox paddingA"
+         ><div class="child1"/><div class="child2"/></div>
+    <div class="flexbox paddingB"
+         ><div class="child1"/><div class="child2"/></div>
+  </body>
+</html>
+
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-010-ref.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-010-ref.html
new file mode 100644
index 0000000000..e488416412
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-010-ref.html
@@ -0,0 +1,94 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>CSS Reftest Reference</title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: block;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 150px;
+    width: 500px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+
+  .container > .hl, .container > .hr {
+    /* In the testcase, these items are stretched vertically
+       via the default "align-self:stretch" behavior, and because
+       they have a height of "auto".
+       (The rest of the items have a non-auto height from "inline-size"
+       and their vertical writing-mode, so those ones do not stretch.) */
+    height: 118px;
+  }
+
+  .container.hl > * { float: left; }
+  .container.hr > * { float: right; }
+
+  </style>
+</head>
+<body>
+
+<div class="container hl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container hl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container hr">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container hr">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-010.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-010.html
new file mode 100644
index 0000000000..55c50e850f
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-010.html
@@ -0,0 +1,88 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>
+    CSS Test: Testing a mix of flex items with various values for
+    'writing-mode' / 'direction' in a horizontal row-oriented flex container.
+  </title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#flex-direction-property">
+  <link rel="help" href="https://www.w3.org/TR/css-writing-modes-3/#propdef-writing-mode">
+  <link rel="match" href="flexbox-writing-mode-010-ref.html">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: flex;
+    flex-direction: row;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 150px;
+    width: 500px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+  </style>
+</head>
+<body>
+
+<div class="container hl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container hl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container hr">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container hr">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-011-ref.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-011-ref.html
new file mode 100644
index 0000000000..8b926490b6
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-011-ref.html
@@ -0,0 +1,97 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>CSS Reftest Reference</title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: block;
+    float: left;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 500px;
+    width: 150px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+
+    /* This really floats to top ('logical left'), since all the containers
+       have a vertical writing mode. */
+    float: left;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+
+
+  .container > .vl,     .container > .vr,
+  .container > .vl_rtl, .container > .vr_rtl {
+    /* In the testcase, these items are stretched horizontally
+       via the default "align-self:stretch" behavior, and because
+       they have a width of "auto".
+       (The rest of the items have a non-auto width from "inline-size"
+       and their horizontal writing-mode, so those ones do not stretch.) */
+    width: 126px;
+  }
+  </style>
+</head>
+<body>
+
+<div class="container vl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container vr">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vr">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-011.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-011.html
new file mode 100644
index 0000000000..6114e19275
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-011.html
@@ -0,0 +1,89 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>
+    CSS Test: Testing a mix of flex items with various values for
+    'writing-mode' / 'direction' in a vertical row-oriented flex container.
+  </title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#flex-direction-property">
+  <link rel="help" href="https://www.w3.org/TR/css-writing-modes-3/#propdef-writing-mode">
+  <link rel="match" href="flexbox-writing-mode-011-ref.html">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: flex;
+    flex-direction: row;
+    float: left;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 500px;
+    width: 150px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+  </style>
+</head>
+<body>
+
+<div class="container vl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container vr">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vr">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-012-ref.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-012-ref.html
new file mode 100644
index 0000000000..c03358be42
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-012-ref.html
@@ -0,0 +1,97 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>CSS Reftest Reference</title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: block;
+    float: left;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 500px;
+    width: 150px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+
+    /* This really floats to top ('logical left'), since all the containers
+       have a vertical writing mode. */
+    float: right;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+
+
+  .container > .vl,     .container > .vr,
+  .container > .vl_rtl, .container > .vr_rtl {
+    /* In the testcase, these items are stretched horizontally
+       via the default "align-self:stretch" behavior, and because
+       they have a width of "auto".
+       (The rest of the items have a non-auto width from "inline-size"
+       and their horizontal writing-mode, so those ones do not stretch.) */
+    width: 126px;
+  }
+  </style>
+</head>
+<body>
+
+<div class="container vl_rtl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vl_rtl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container vr_rtl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vr_rtl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-012.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-012.html
new file mode 100644
index 0000000000..a7b05f178d
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-012.html
@@ -0,0 +1,90 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>
+    CSS Test: Testing a mix of flex items with various values for
+    'writing-mode' / 'direction' in a vertical row-oriented flex container
+    with 'direction' flipped.
+  </title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#flex-direction-property">
+  <link rel="help" href="https://www.w3.org/TR/css-writing-modes-3/#propdef-writing-mode">
+  <link rel="match" href="flexbox-writing-mode-012-ref.html">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: flex;
+    flex-direction: row;
+    float: left;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 500px;
+    width: 150px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+  </style>
+</head>
+<body>
+
+<div class="container vl_rtl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vl_rtl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container vr_rtl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vr_rtl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-013-ref.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-013-ref.html
new file mode 100644
index 0000000000..2a27003a5c
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-013-ref.html
@@ -0,0 +1,98 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>CSS Reftest Reference</title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: block;
+    float: left;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 500px;
+    width: 150px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  nocollapse {
+    /* special element to disable margin-collapsing */
+    display: block;
+    overflow: hidden;
+    height: 0;
+  }
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+
+  .container > .vl,     .container > .vr,
+  .container > .vl_rtl, .container > .vr_rtl {
+    /* In the testcase, these items are stretched horizontally
+       via the default "align-self:stretch" behavior, and because
+       they have a width of "auto".
+       (The rest of the items have a non-auto width from "inline-size"
+       and their horizontal writing-mode, so those ones do not stretch.) */
+    width: 126px;
+  }
+  </style>
+</head>
+<body>
+
+<div class="container hl">
+  <span class="hl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hl big">d e</span>        <nocollapse></nocollapse>
+  <span class="hr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vl big">d e</span>        <nocollapse></nocollapse>
+</div>
+<div class="container hl">
+  <span class="vr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vl_rtl big">d e</span>    <nocollapse></nocollapse>
+  <span class="vr_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vr_rtl big">d e</span>    <nocollapse></nocollapse>
+</div>
+<div class="container hr">
+  <span class="hl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hl big">d e</span>        <nocollapse></nocollapse>
+  <span class="hr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vl big">d e</span>        <nocollapse></nocollapse>
+</div>
+<div class="container hr">
+  <span class="vr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vl_rtl big">d e</span>    <nocollapse></nocollapse>
+  <span class="vr_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vr_rtl big">d e</span>    <nocollapse></nocollapse>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-013.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-013.html
new file mode 100644
index 0000000000..a5d483c442
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-013.html
@@ -0,0 +1,89 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>
+    CSS Test: Testing a mix of flex items with various values for
+    'writing-mode' / 'direction' in a horizontal column-oriented flex container.
+  </title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#flex-direction-property">
+  <link rel="help" href="https://www.w3.org/TR/css-writing-modes-3/#propdef-writing-mode">
+  <link rel="match" href="flexbox-writing-mode-013-ref.html">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: flex;
+    flex-direction: column;
+    float: left;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 500px;
+    width: 150px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+  </style>
+</head>
+<body>
+
+<div class="container hl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container hl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container hr">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container hr">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-014-ref.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-014-ref.html
new file mode 100644
index 0000000000..4e4f5904a1
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-014-ref.html
@@ -0,0 +1,96 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>CSS Reftest Reference</title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: block;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 150px;
+    width: 500px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  nocollapse {
+    /* special element to disable margin-collapsing */
+    display: block;
+    overflow: hidden;
+    height: 0;
+  }
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+
+  .container > .hl, .container > .hr {
+    /* In the testcase, these items are stretched vertically
+       via the default "align-self:stretch" behavior, and because
+       they have a height of "auto".
+       (The rest of the items have a non-auto height from "inline-size"
+       and their vertical writing-mode, so those ones do not stretch.) */
+    height: 118px;
+  }
+  </style>
+</head>
+<body>
+
+<div class="container vl">
+  <span class="hl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hl big">d e</span>        <nocollapse></nocollapse>
+  <span class="hr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vl big">d e</span>        <nocollapse></nocollapse>
+</div>
+<div class="container vl">
+  <span class="vr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vl_rtl big">d e</span>    <nocollapse></nocollapse>
+  <span class="vr_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vr_rtl big">d e</span>    <nocollapse></nocollapse>
+</div>
+<div class="container vr">
+  <span class="hl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hl big">d e</span>        <nocollapse></nocollapse>
+  <span class="hr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vl big">d e</span>        <nocollapse></nocollapse>
+</div>
+<div class="container vr">
+  <span class="vr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vl_rtl big">d e</span>    <nocollapse></nocollapse>
+  <span class="vr_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vr_rtl big">d e</span>    <nocollapse></nocollapse>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-014.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-014.html
new file mode 100644
index 0000000000..1a5443279a
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-014.html
@@ -0,0 +1,88 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>
+    CSS Test: Testing a mix of flex items with various values for
+    'writing-mode' / 'direction' in a vertical column-oriented flex container.
+  </title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#flex-direction-property">
+  <link rel="help" href="https://www.w3.org/TR/css-writing-modes-3/#propdef-writing-mode">
+  <link rel="match" href="flexbox-writing-mode-014-ref.html">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: flex;
+    flex-direction: column;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 150px;
+    width: 500px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+  </style>
+</head>
+<body>
+
+<div class="container vl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container vr">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vr">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-015-ref.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-015-ref.html
new file mode 100644
index 0000000000..a236ca11d5
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-015-ref.html
@@ -0,0 +1,96 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>CSS Reftest Reference</title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: block;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 150px;
+    width: 500px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  nocollapse {
+    /* special element to disable margin-collapsing */
+    display: block;
+    overflow: hidden;
+    height: 0;
+  }
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+
+  .container > .hl, .container > .hr {
+    /* In the testcase, these items are stretched vertically
+       via the default "align-self:stretch" behavior, and because
+       they have a height of "auto".
+       (The rest of the items have a non-auto height from "inline-size"
+       and their vertical writing-mode, so those ones do not stretch.) */
+    height: 118px;
+  }
+  </style>
+</head>
+<body>
+
+<div class="container vl_rtl">
+  <span class="hl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hl big">d e</span>        <nocollapse></nocollapse>
+  <span class="hr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vl big">d e</span>        <nocollapse></nocollapse>
+</div>
+<div class="container vl_rtl">
+  <span class="vr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vl_rtl big">d e</span>    <nocollapse></nocollapse>
+  <span class="vr_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vr_rtl big">d e</span>    <nocollapse></nocollapse>
+</div>
+<div class="container vr_rtl">
+  <span class="hl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hl big">d e</span>        <nocollapse></nocollapse>
+  <span class="hr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="hr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vl big">d e</span>        <nocollapse></nocollapse>
+</div>
+<div class="container vr_rtl">
+  <span class="vr small">a b c</span>    <nocollapse></nocollapse>
+  <span class="vr big">d e</span>        <nocollapse></nocollapse>
+  <span class="vl_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vl_rtl big">d e</span>    <nocollapse></nocollapse>
+  <span class="vr_rtl small">a b c</span><nocollapse></nocollapse>
+  <span class="vr_rtl big">d e</span>    <nocollapse></nocollapse>
+</div>
+
+</body>
+</html>
diff --git a/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-015.html b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-015.html
new file mode 100644
index 0000000000..af1f9b8456
--- /dev/null
+++ b/layout/reftests/w3c-css/submitted/flexbox/flexbox-writing-mode-015.html
@@ -0,0 +1,89 @@
+<!DOCTYPE html>
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>
+    CSS Test: Testing a mix of flex items with various values for
+    'writing-mode' / 'direction' in a vertical column-oriented flex container
+    with 'direction' flipped.
+  </title>
+  <meta charset="utf-8">
+  <link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
+  <link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#flex-direction-property">
+  <link rel="help" href="https://www.w3.org/TR/css-writing-modes-3/#propdef-writing-mode">
+  <link rel="match" href="flexbox-writing-mode-015-ref.html">
+  <link rel="stylesheet" type="text/css" href="support/ahem.css">
+  <style>
+  .container {
+    display: flex;
+    flex-direction: column;
+    border: 2px solid purple;
+    padding: 2px;
+    margin-bottom: 2em;
+    height: 150px;
+    width: 500px;
+  }
+
+  span {
+    display: block;
+    background: lightgrey;
+    border: 2px solid black;
+    /* If browser supports it, signal the inline direction with border color: */
+    border-block-start-color: orange;
+    border-inline-start-color: lime;
+
+    margin: 11px 13px 17px 7px;
+    inline-size: 6px;
+  }
+
+  .small { font: 12px Ahem; }
+  .big   { font: 20px Ahem; }
+
+  .hl  { writing-mode: horizontal-tb;  direction: ltr; }
+  .hr  { writing-mode: horizontal-tb;  direction: rtl; }
+  .vl  { writing-mode: vertical-lr;    direction: ltr; }
+  .vr  { writing-mode: vertical-rl;    direction: ltr; }
+  .vl_rtl { writing-mode: vertical-lr; direction: rtl; }
+  .vr_rtl { writing-mode: vertical-rl; direction: rtl; }
+  </style>
+</head>
+<body>
+
+<div class="container vl_rtl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vl_rtl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+<div class="container vr_rtl">
+  <span class="hl small">a b c</span>
+  <span class="hl big">d e</span>
+  <span class="hr small">a b c</span>
+  <span class="hr big">d e</span>
+  <span class="vl small">a b c</span>
+  <span class="vl big">d e</span>
+</div>
+<div class="container vr_rtl">
+  <span class="vr small">a b c</span>
+  <span class="vr big">d e</span>
+  <span class="vl_rtl small">a b c</span>
+  <span class="vl_rtl big">d e</span>
+  <span class="vr_rtl small">a b c</span>
+  <span class="vr_rtl big">d e</span>
+</div>
+
+</body>
+</html>