--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -10621,16 +10621,24 @@ FrameWantsToBeInAnonymousItem(const nsIF
              containerType == nsGkAtoms::gridContainerFrame);
 
   // Any line-participant frames (e.g. text) definitely want to be wrapped in
   // an anonymous flex/grid item.
   if (aFrame->IsFrameOfType(nsIFrame::eLineParticipant)) {
     return true;
   }
 
+  // If the container is a -webkit-box/-webkit-inline-box, then placeholders
+  // also need to be wrapped, for compatibility.
+  if (containerType == nsGkAtoms::flexContainerFrame &&
+      aContainerFrame->HasAnyStateBits(NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX) &&
+      aFrame->GetType() == nsGkAtoms::placeholderFrame) {
+    return true;
+  }
+
   return false;
 }
 #endif
 
 static void
 VerifyGridFlexContainerChildren(nsIFrame* aParentFrame,
                                 const nsFrameList& aChildren)
 {
@@ -12767,20 +12775,30 @@ nsCSSFrameConstructor::FrameConstruction
   NeedsAnonFlexOrGridItem(const nsFrameConstructorState& aState,
                           bool aIsWebkitBox)
 {
   if (mFCData->mBits & FCDATA_IS_LINE_PARTICIPANT) {
     // This will be an inline non-replaced box.
     return true;
   }
 
-  if (aIsWebkitBox &&
-      mStyleContext->StyleDisplay()->IsInlineOutsideStyle()) {
-    // In a -webkit-box, all inline-level content gets wrapped in an anon item.
-    return true;
+  if (aIsWebkitBox) {
+    if (mStyleContext->StyleDisplay()->IsInlineOutsideStyle()) {
+      // In a -webkit-box, all inline-level content gets wrapped in anon item.
+      return true;
+    }
+    if (!(mFCData->mBits & FCDATA_DISALLOW_OUT_OF_FLOW) &&
+        aState.GetGeometricParent(mStyleContext->StyleDisplay(), nullptr)) {
+      // We're abspos or fixedpos, which means we'll spawn a placeholder which
+      // (because our container is a -webkit-box) we'll need to wrap in an
+      // anonymous flex item.  So, we just treat _this_ frame as if _it_ needs
+      // to be wrapped in an anonymous flex item, and then when we spawn the
+      // placeholder, it'll end up in the right spot.
+      return true;
+    }
   }
 
   return false;
 }
 
 inline bool
 nsCSSFrameConstructor::FrameConstructionItemList::
 Iterator::SkipItemsThatNeedAnonFlexOrGridItem(