--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -565,17 +565,25 @@ nsBlockFrame::GetAdditionalChildListName
   default:
     return nsnull;
   }
 }
 
 /* virtual */ PRBool
 nsBlockFrame::IsContainingBlock() const
 {
-  return PR_TRUE;
+  // The block wrappers we use to wrap blocks inside inlines aren't
+  // described in the CSS spec.  We need to make them not be containing
+  // blocks for the cases where we care about the 'direction' property
+  // of an element's containing block.
+  // Since the parent of such a block is either a normal block or
+  // another such pseudo, this shouldn't cause anything bad to happen.
+  nsIAtom *pseudoType = GetStyleContext()->GetPseudoType();
+  return pseudoType != nsCSSAnonBoxes::mozAnonymousBlock &&
+         pseudoType != nsCSSAnonBoxes::mozAnonymousPositionedBlock;
 }
 
 /* virtual */ PRBool
 nsBlockFrame::IsFloatContainingBlock() const
 {
   return PR_TRUE;
 }
 

--- a/layout/reftests/box-properties/reftest.list
+++ b/layout/reftests/box-properties/reftest.list
@@ -13,10 +13,10 @@
 == width-special-values-cell-auto.html width-special-values-cell-auto-ref.html
 == width-special-values-cell-fixed.html width-special-values-cell-fixed-ref.html
 == box-sizing-1.html box-sizing-1-ref.html
 == box-sizing-2.html box-sizing-2-ref.html
 == box-sizing-3.html box-sizing-1-ref.html
 == box-sizing-4.html box-sizing-4-ref.html
 == abspos-non-replaced-width-offset-margin.html abspos-non-replaced-width-offset-margin-ref.html
 == abspos-replaced-width-offset-margin.html abspos-replaced-width-offset-margin-ref.html
-fails == CSS21-t100303.xhtml CSS21-t100303-ref.xhtml # bug 423306
+== CSS21-t100303.xhtml CSS21-t100303-ref.xhtml
 == CSS21-t100303-simple.xhtml CSS21-t100303-ref.xhtml