--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -1190,16 +1190,143 @@ IsOrderLEQ(nsIFrame* aFrame1,
   bool isInLegacyBox = nsFlexContainerFrame::IsLegacyBox(aFrame1->GetParent());
 
   int32_t order1 = GetOrderOrBoxOrdinalGroup(aFrame1, isInLegacyBox);
   int32_t order2 = GetOrderOrBoxOrdinalGroup(aFrame2, isInLegacyBox);
 
   return order1 <= order2;
 }
 
+uint8_t
+SimplifyAlignOrJustifyContentForOneItem(uint16_t aAlignmentVal,
+                                        bool aIsAlign)
+{
+  // Mask away any explicit fallback, to get the main (non-fallback) part of
+  // the specified value:
+  uint16_t specified = aAlignmentVal & NS_STYLE_ALIGN_ALL_BITS;
+
+  // XXX strip off <overflow-position> bits until we implement it (bug 1311892)
+  specified &= ~NS_STYLE_ALIGN_FLAG_BITS;
+
+  // FIRST: handle a special-case for "justify-content:stretch" (or equivalent),
+  // which requires that we ignore any author-provided explicit fallback value.
+  if (specified == NS_STYLE_ALIGN_NORMAL) {
+    // In a flex container, *-content: "'normal' behaves as 'stretch'".
+    // Do that conversion early, so it benefits from our 'stretch' special-case.
+    // https://drafts.csswg.org/css-align-3/#distribution-flex
+    specified = NS_STYLE_ALIGN_STRETCH;
+  }
+  if (!aIsAlign && specified == NS_STYLE_ALIGN_STRETCH) {
+    // In a flex container, in "justify-content Axis: [...] 'stretch' behaves
+    // as 'flex-start' (ignoring the specified fallback alignment, if any)."
+    // https://drafts.csswg.org/css-align-3/#distribution-flex
+    // So, we just directly return 'flex-start', & ignore explicit fallback..
+    return NS_STYLE_ALIGN_FLEX_START;
+  }
+
+  // Now check for an explicit fallback value (and if it's present, use it).
+  uint16_t explicitFallback = aAlignmentVal >> NS_STYLE_ALIGN_ALL_SHIFT;
+  if (explicitFallback) {
+    // XXX strip off <overflow-position> bits until we implement it
+    // (bug 1311892)
+    explicitFallback &= ~NS_STYLE_ALIGN_FLAG_BITS;
+    return explicitFallback;
+  }
+
+  // There's no explicit fallback. Use the implied fallback values for
+  // space-{between,around,evenly} (since those values only make sense with
+  // multiple alignment subjects), and otherwise just use the specified value:
+  switch (specified) {
+    case NS_STYLE_ALIGN_SPACE_BETWEEN:
+      return NS_STYLE_ALIGN_START;
+    case NS_STYLE_ALIGN_SPACE_AROUND:
+    case NS_STYLE_ALIGN_SPACE_EVENLY:
+      return NS_STYLE_ALIGN_CENTER;
+    default:
+      return specified;
+  }
+}
+
+uint16_t
+nsFlexContainerFrame::CSSAlignmentForAbsPosChild(
+  const ReflowInput& aChildRI,
+  LogicalAxis aLogicalAxis) const
+{
+  WritingMode wm = GetWritingMode();
+  const FlexboxAxisTracker
+    axisTracker(this, wm, AxisTrackerFlags::eAllowBottomToTopChildOrdering);
+
+  // If we're row-oriented and the caller is asking about our inline axis (or
+  // alternately, if we're column-oriented and the caller is asking about our
+  // block axis), then the caller is really asking about our *main* axis.
+  // Otherwise, the caller is asking about our cross axis.
+  const bool isMainAxis = (axisTracker.IsRowOriented() ==
+                           (aLogicalAxis == eLogicalAxisInline));
+  const nsStylePosition* containerStylePos = StylePosition();
+  const bool isAxisReversed = isMainAxis ? axisTracker.IsMainAxisReversed()
+                                         : axisTracker.IsCrossAxisReversed();
+
+  uint8_t alignment;
+  if (isMainAxis) {
+    alignment = SimplifyAlignOrJustifyContentForOneItem(
+                  containerStylePos->mJustifyContent,
+                  /*aIsAlign = */false);
+  } else {
+    const uint8_t alignContent = SimplifyAlignOrJustifyContentForOneItem(
+                                   containerStylePos->mAlignContent,
+                                   /*aIsAlign = */true);
+    if (NS_STYLE_FLEX_WRAP_NOWRAP != containerStylePos->mFlexWrap &&
+        alignContent != NS_STYLE_ALIGN_STRETCH) {
+      // Multi-line, align-content isn't stretch --> align-content determines
+      // this child's alignment in the cross axis.
+      alignment = alignContent;
+    } else {
+      // Single-line, or multi-line but the (one) line stretches to fill
+      // container. Respect align-self.
+      alignment = aChildRI.mStylePosition->UsedAlignSelf(nullptr);
+      // XXX strip off <overflow-position> bits until we implement it
+      // (bug 1311892)
+      alignment &= ~NS_STYLE_ALIGN_FLAG_BITS;
+
+      if (alignment == NS_STYLE_ALIGN_NORMAL) {
+        // "the 'normal' keyword behaves as 'start' on replaced
+        // absolutely-positioned boxes, and behaves as 'stretch' on all other
+        // absolutely-positioned boxes."
+        // https://drafts.csswg.org/css-align/#align-abspos
+        alignment = aChildRI.mFrame->IsFrameOfType(nsIFrame::eReplaced) ?
+          NS_STYLE_ALIGN_START : NS_STYLE_ALIGN_STRETCH;
+      }
+    }
+  }
+
+  // Resolve flex-start, flex-end, auto, left, right, baseline, last-baseline;
+  if (alignment == NS_STYLE_ALIGN_FLEX_START) {
+    alignment = isAxisReversed ? NS_STYLE_ALIGN_END : NS_STYLE_ALIGN_START;
+  } else if (alignment == NS_STYLE_ALIGN_FLEX_END) {
+    alignment = isAxisReversed ? NS_STYLE_ALIGN_START : NS_STYLE_ALIGN_END;
+  } else if (alignment == NS_STYLE_ALIGN_AUTO) {
+    alignment = NS_STYLE_ALIGN_START;
+  } else if (alignment == NS_STYLE_ALIGN_LEFT ||
+             alignment == NS_STYLE_ALIGN_RIGHT) {
+    if (aLogicalAxis == eLogicalAxisInline) {
+      const bool isLeft = (alignment == NS_STYLE_ALIGN_LEFT);
+      alignment = (isLeft == wm.IsBidiLTR()) ? NS_STYLE_ALIGN_START
+                                             : NS_STYLE_ALIGN_END;
+    } else {
+      alignment = NS_STYLE_ALIGN_START;
+    }
+  } else if (alignment == NS_STYLE_ALIGN_BASELINE) {
+    alignment = NS_STYLE_ALIGN_START;
+  } else if (alignment == NS_STYLE_ALIGN_LAST_BASELINE) {
+    alignment = NS_STYLE_ALIGN_END;
+  }
+
+  return alignment;
+}
+
 bool
 nsFlexContainerFrame::IsHorizontal()
 {
   const FlexboxAxisTracker axisTracker(this, GetWritingMode());
   return axisTracker.IsMainAxisHorizontal();
 }
 
 UniquePtr<FlexItem>

--- a/layout/generic/nsFlexContainerFrame.h
+++ b/layout/generic/nsFlexContainerFrame.h
@@ -74,16 +74,21 @@ public:
 
   virtual nsIAtom* GetType() const override;
 #ifdef DEBUG_FRAME_DUMP
   virtual nsresult GetFrameName(nsAString& aResult) const override;
 #endif
 
   nscoord GetLogicalBaseline(mozilla::WritingMode aWM) const override;
 
+  // nsContainerFrame overrides
+  uint16_t CSSAlignmentForAbsPosChild(
+            const ReflowInput& aChildRI,
+            mozilla::LogicalAxis aLogicalAxis) const override;
+
   // Flexbox-specific public methods
   bool IsHorizontal();
 
   /**
     * Helper function to calculate packing space and initial offset of alignment
     * subjects in MainAxisPositionTracker() and CrossAxisPositionTracker() for
     * space-between, space-around, and space-evenly.
     *
@@ -274,18 +279,16 @@ protected:
    * out-of-flow (OOF) frames -- we can't compute that until we've reflowed the
    * OOF, because (depending on the CSS Align properties) the static position
    * may be influenced by the OOF's size.  So for now, we just co-opt the
    * placeholder to store the flex container's logical content-box origin, and
    * we defer to nsAbsoluteContainingBlock to determine the OOF's actual static
    * position (using this origin, the OOF's size, and the CSS Align
    * properties).
    *
-   * XXXdholbert The nsAbsoluteContainingBlock stuff is coming in bug 1269046.
-   *
    * @param aPresContext       The presentation context being used in reflow.
    * @param aReflowInput       The flex container's reflow input.
    * @param aPlaceholders      An array of all the flex container's
    *                           nsPlaceholderFrame children.
    * @param aContentBoxOrigin  The flex container's logical content-box
    *                           origin (in its own coordinate space).
    * @param aContainerSize     The flex container's size (required by some
    *                           reflow methods to interpret positions correctly).

--- a/layout/reftests/flexbox/reftest.list
+++ b/layout/reftests/flexbox/reftest.list
@@ -68,22 +68,22 @@ fuzzy-if(skiaContent,3,10) == flexbox-dy
 == flexbox-float-2a.xhtml  flexbox-float-2-ref.xhtml
 == flexbox-float-2b.xhtml  flexbox-float-2-ref.xhtml
 
 # Tests for the order in which we paint flex items
 fails == flexbox-paint-ordering-3.html  flexbox-paint-ordering-3-ref.html # bug 874718
 
 # Tests for handling of absolutely/fixed/relatively-positioned flex items.
 == flexbox-position-absolute-1.xhtml  flexbox-position-absolute-1-ref.xhtml
-fails == flexbox-position-absolute-2.xhtml  flexbox-position-absolute-2-ref.xhtml # bug 1269046
+== flexbox-position-absolute-2.xhtml  flexbox-position-absolute-2-ref.xhtml
 == flexbox-position-absolute-3.xhtml  flexbox-position-absolute-3-ref.xhtml
 == flexbox-position-absolute-4.xhtml  flexbox-position-absolute-4-ref.xhtml
 == flexbox-position-fixed-3.xhtml     flexbox-position-fixed-3-ref.xhtml
 fuzzy-if(Android,16,400) == flexbox-position-fixed-1.xhtml     flexbox-position-fixed-1-ref.xhtml
-fuzzy-if(Android,16,400) fails == flexbox-position-fixed-2.xhtml     flexbox-position-fixed-2-ref.xhtml # bug 1269046
+fuzzy-if(Android,16,400) == flexbox-position-fixed-2.xhtml     flexbox-position-fixed-2-ref.xhtml
 == flexbox-position-fixed-3.xhtml     flexbox-position-fixed-3-ref.xhtml
 == flexbox-position-fixed-4.xhtml     flexbox-position-fixed-4-ref.xhtml
 
 # Tests for inline content in a flexbox that gets wrapped in an anonymous block
 fails == flexbox-inlinecontent-horiz-1a.xhtml flexbox-inlinecontent-horiz-1-ref.xhtml # reference case rendering is incorrect; bug 858333
 fails == flexbox-inlinecontent-horiz-1b.xhtml flexbox-inlinecontent-horiz-1-ref.xhtml # reference case rendering is incorrect; bug 858333
 == flexbox-inlinecontent-horiz-2.xhtml  flexbox-inlinecontent-horiz-2-ref.xhtml
 == flexbox-inlinecontent-horiz-3a.xhtml flexbox-inlinecontent-horiz-3-ref.xhtml