diff --git a/content/base/public/DirectionalityUtils.h b/content/base/public/DirectionalityUtils.h
new file mode 100644
index 000000000000..195f7009de2f
--- /dev/null
+++ b/content/base/public/DirectionalityUtils.h
@@ -0,0 +1,55 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DirectionalityUtils_h___
+#define DirectionalityUtils_h___
+
+class nsIContent;
+class nsIDocument;
+class nsINode;
+
+namespace mozilla {
+namespace dom {
+class Element;
+} // namespace dom
+} // namespace mozilla
+
+namespace mozilla {
+
+namespace directionality {
+
+enum Directionality {
+  eDir_NotSet = 0,
+  eDir_RTL    = 1,
+  eDir_LTR    = 2
+};
+
+/**
+ * Set the directionality of an element according to the algorithm defined at
+ * http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#the-directionality,
+ * not including elements with auto direction.
+ *
+ * @return the directionality that the element was set to
+ */
+Directionality RecomputeDirectionality(mozilla::dom::Element* aElement,
+                                       bool aNotify = true);
+
+/**
+ * Set the directionality of any descendants of a node that do not themselves
+ * have a dir attribute.
+ * For performance reasons we walk down the descendant tree in the rare case
+ * of setting the dir attribute, rather than walking up the ancestor tree in
+ * the much more common case of getting the element's directionality.
+ */
+void SetDirectionalityOnDescendants(mozilla::dom::Element* aElement, 
+                                    Directionality aDir,
+                                    bool aNotify = true);
+
+} // end namespace directionality
+
+} // end namespace mozilla
+
+#endif /* DirectionalityUtils_h___ */
diff --git a/content/base/public/Element.h b/content/base/public/Element.h
index 4d1b51b1acbd..c76bfe03ca95 100644
--- a/content/base/public/Element.h
+++ b/content/base/public/Element.h
@@ -10,6 +10,7 @@
 #include "mozilla/dom/FragmentOrElement.h" // for base class
 #include "nsChangeHint.h"                  // for enum
 #include "nsEventStates.h"                 // for member
+#include "mozilla/dom/DirectionalityUtils.h"
 
 class nsEventStateManager;
 class nsFocusManager;
@@ -216,6 +217,54 @@ public:
    */
   virtual nsIAtom *GetClassAttributeName() const = 0;
 
+  inline mozilla::directionality::Directionality GetDirectionality() const {
+    if (HasFlag(NODE_HAS_DIRECTION_RTL)) {
+      return mozilla::directionality::eDir_RTL;
+    }
+
+    if (HasFlag(NODE_HAS_DIRECTION_LTR)) {
+      return mozilla::directionality::eDir_LTR;
+    }
+
+    return mozilla::directionality::eDir_NotSet;
+  }
+
+  inline void SetDirectionality(mozilla::directionality::Directionality aDir,
+                                bool aNotify) {
+    UnsetFlags(NODE_ALL_DIRECTION_FLAGS);
+    if (!aNotify) {
+      RemoveStatesSilently(DIRECTION_STATES);
+    }
+
+    switch (aDir) {
+      case (mozilla::directionality::eDir_RTL):
+        SetFlags(NODE_HAS_DIRECTION_RTL);
+        if (!aNotify) {
+          AddStatesSilently(NS_EVENT_STATE_RTL);
+        }
+        break;
+
+      case(mozilla::directionality::eDir_LTR):
+        SetFlags(NODE_HAS_DIRECTION_LTR);
+        if (!aNotify) {
+          AddStatesSilently(NS_EVENT_STATE_LTR);
+        }
+        break;
+
+      default:
+        break;
+    }
+
+    /* 
+     * Only call UpdateState if we need to notify, because we call
+     * SetDirectionality for every element, and UpdateState is very very slow
+     * for some elements.
+     */
+    if (aNotify) {
+      UpdateState(true);
+    }
+  }
+
 protected:
   /**
    * Method to get the _intrinsic_ content state of this element.  This is the
diff --git a/content/base/public/Makefile.in b/content/base/public/Makefile.in
index 49c521897795..5d92d5876f2b 100644
--- a/content/base/public/Makefile.in
+++ b/content/base/public/Makefile.in
@@ -48,6 +48,7 @@ $(NULL)
 EXPORTS_NAMESPACES = mozilla/dom mozilla
 
 EXPORTS_mozilla/dom = \
+		DirectionalityUtils.h \
 		Element.h \
 		FragmentOrElement.h \
 		FromParser.h \
diff --git a/content/base/public/nsIDocument.h b/content/base/public/nsIDocument.h
index 16307c8d5cf1..0845a2954312 100644
--- a/content/base/public/nsIDocument.h
+++ b/content/base/public/nsIDocument.h
@@ -23,6 +23,7 @@
 #include "nsPIDOMWindow.h"               // for use in inline functions
 #include "nsPropertyTable.h"             // for member
 #include "nsTHashtable.h"                // for member
+#include "mozilla/dom/DirectionalityUtils.h"
 
 class imgIRequest;
 class nsAString;
@@ -397,6 +398,10 @@ public:
   {
     mBidiOptions = aBidiOptions;
   }
+
+  inline mozilla::directionality::Directionality GetDocumentDirectionality() {
+    return mDirectionality;
+  }
   
   /**
    * Access HTTP header data (this may also get set from other
@@ -1853,6 +1858,9 @@ protected:
   // defined in nsBidiUtils.h
   PRUint32 mBidiOptions;
 
+  // The root directionality of this document.
+  mozilla::directionality::Directionality mDirectionality;
+
   nsCString mContentLanguage;
 private:
   nsCString mContentType;
diff --git a/content/base/public/nsINode.h b/content/base/public/nsINode.h
index 0d79acf33736..189d64265507 100644
--- a/content/base/public/nsINode.h
+++ b/content/base/public/nsINode.h
@@ -145,8 +145,16 @@ enum {
   // Set if the node has had :hover selectors matched against it
   NODE_HAS_RELEVANT_HOVER_RULES = 0x00080000U,
 
+  // Set if the node has right-to-left directionality
+  NODE_HAS_DIRECTION_RTL        = 0x00100000U,
+
+  // Set if the node has left-to-right directionality
+  NODE_HAS_DIRECTION_LTR        = 0x00200000U,
+
+  NODE_ALL_DIRECTION_FLAGS      = NODE_HAS_DIRECTION_LTR | NODE_HAS_DIRECTION_RTL,
+
   // Remaining bits are node type specific.
-  NODE_TYPE_SPECIFIC_BITS_OFFSET =        20
+  NODE_TYPE_SPECIFIC_BITS_OFFSET =        22
 };
 
 /**
@@ -1279,6 +1287,8 @@ private:
     NodeIsContent,
     // Set if the node has animations or transitions
     ElementHasAnimations,
+    // Set if node has a dir attribute with a valid value (ltr or rtl)
+    NodeHasValidDirAttribute,
     // Guard value
     BooleanFlagCount
   };
@@ -1346,6 +1356,9 @@ public:
   void ClearPointerLock() { ClearBoolFlag(ElementHasPointerLock); }
   bool MayHaveAnimations() { return GetBoolFlag(ElementHasAnimations); }
   void SetMayHaveAnimations() { SetBoolFlag(ElementHasAnimations); }
+  void SetHasValidDir() { SetBoolFlag(NodeHasValidDirAttribute); }
+  void ClearHasValidDir() { ClearBoolFlag(NodeHasValidDirAttribute); }
+  bool HasValidDir() const { return GetBoolFlag(NodeHasValidDirAttribute); }
 protected:
   void SetParentIsContent(bool aValue) { SetBoolFlag(ParentIsContent, aValue); }
   void SetInDocument() { SetBoolFlag(IsInDocument); }
diff --git a/content/base/src/DirectionalityUtils.cpp b/content/base/src/DirectionalityUtils.cpp
new file mode 100644
index 000000000000..8523e25a333a
--- /dev/null
+++ b/content/base/src/DirectionalityUtils.cpp
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 sw=2 et tw=78: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/dom/DirectionalityUtils.h"
+#include "nsINode.h"
+#include "nsIContent.h"
+#include "nsIDocument.h"
+#include "mozilla/dom/Element.h"
+#include "nsIDOMNodeFilter.h"
+#include "nsTreeWalker.h"
+#include "nsIDOMHTMLDocument.h"
+
+
+namespace mozilla {
+
+namespace directionality {
+
+typedef mozilla::dom::Element Element;
+
+Directionality
+RecomputeDirectionality(Element* aElement, bool aNotify)
+{
+  Directionality dir = eDir_LTR;
+
+  if (aElement->HasValidDir()) {
+    dir = aElement->GetDirectionality();
+  } else {
+    Element* parent = aElement->GetElementParent();
+    if (parent) {
+      // If the element doesn't have an explicit dir attribute with a valid
+      // value, the directionality is the same as the parent element (but
+      // don't propagate the parent directionality if it isn't set yet).
+      Directionality parentDir = parent->GetDirectionality();
+      if (parentDir != eDir_NotSet) {
+        dir = parentDir;
+      }
+    } else {
+      // If there is no parent element, the directionality is the same as the
+      // document direction.
+      Directionality documentDir =
+        aElement->OwnerDoc()->GetDocumentDirectionality();
+      if (documentDir != eDir_NotSet) {
+        dir = documentDir;
+      }
+    }
+    
+    aElement->SetDirectionality(dir, aNotify);
+  }
+  return dir;
+}
+
+void
+SetDirectionalityOnDescendants(Element* aElement, Directionality aDir,
+                               bool aNotify)
+{
+  for (nsIContent* child = aElement->GetFirstChild(); child; ) {
+    if (!child->IsElement()) {
+      child = child->GetNextNode(aElement);
+      continue;
+    }
+
+    Element* element = child->AsElement();
+    if (element->HasValidDir()) {
+      child = child->GetNextNonChildNode(aElement);
+      continue;
+    }
+    element->SetDirectionality(aDir, aNotify);
+    child = child->GetNextNode(aElement);
+  }
+}
+
+} // end namespace directionality
+
+} // end namespace mozilla
+
diff --git a/content/base/src/Makefile.in b/content/base/src/Makefile.in
index fdec933a82b7..c7c1d95097a6 100644
--- a/content/base/src/Makefile.in
+++ b/content/base/src/Makefile.in
@@ -53,6 +53,7 @@ LOCAL_INCLUDES = \
 		$(NULL)
 
 CPPSRCS		= \
+		DirectionalityUtils.cpp \
 		nsAtomListUtils.cpp \
 		nsAttrAndChildArray.cpp \
 		nsAttrValue.cpp \
diff --git a/content/base/src/nsDocument.cpp b/content/base/src/nsDocument.cpp
index 643fb2e05b44..5a1f4c200094 100644
--- a/content/base/src/nsDocument.cpp
+++ b/content/base/src/nsDocument.cpp
@@ -91,6 +91,7 @@
 #include "nsXMLEventsManager.h"
 
 #include "nsBidiUtils.h"
+#include "mozilla/dom/DirectionalityUtils.h"
 
 #include "nsIDOMUserDataHandler.h"
 #include "nsIDOMXPathEvaluator.h"
@@ -170,6 +171,7 @@
 
 using namespace mozilla;
 using namespace mozilla::dom;
+using namespace mozilla::directionality;
 
 typedef nsTArray<Link*> LinkArray;
 
@@ -1510,6 +1512,7 @@ nsIDocument::nsIDocument()
     mAllowDNSPrefetch(true),
     mIsBeingUsedAsImage(false),
     mHasLinksToUpdate(false),
+    mDirectionality(eDir_LTR),
     mPartID(0)
 {
   SetInDocument();
@@ -5583,6 +5586,15 @@ nsDocument::SetDir(const nsAString& aDirection)
           // No presentation; just set it on ourselves
           SetBidiOptions(options);
         }
+        Directionality dir = elt->mValue == IBMBIDI_TEXTDIRECTION_RTL ?
+                               eDir_RTL : eDir_LTR;
+        SetDocumentDirectionality(dir);
+        // Set the directionality of the root element and its descendants, if any
+        Element* rootElement = GetRootElement();
+        if (rootElement) {
+          rootElement->SetDirectionality(dir, true);
+          SetDirectionalityOnDescendants(rootElement, dir);
+        }
       }
 
       break;
diff --git a/content/base/src/nsDocument.h b/content/base/src/nsDocument.h
index bbe012601d80..3ef75fda92b2 100644
--- a/content/base/src/nsDocument.h
+++ b/content/base/src/nsDocument.h
@@ -1048,6 +1048,12 @@ protected:
   nsIContent* GetFirstBaseNodeWithHref();
   nsresult SetFirstBaseNodeWithHref(nsIContent *node);
 
+  inline void
+  SetDocumentDirectionality(mozilla::directionality::Directionality aDir)
+  {
+    mDirectionality = aDir;
+  }
+
   // Get the first <title> element with the given IsNodeOfType type, or
   // return null if there isn't one
   nsIContent* GetTitleContent(PRUint32 aNodeType);
diff --git a/content/base/src/nsGenericElement.cpp b/content/base/src/nsGenericElement.cpp
index e591e7ed11af..a421bd5299f0 100644
--- a/content/base/src/nsGenericElement.cpp
+++ b/content/base/src/nsGenericElement.cpp
@@ -51,6 +51,7 @@
 #include "nsIDOMMutationEvent.h"
 #include "nsMutationEvent.h"
 #include "nsNodeUtils.h"
+#include "mozilla/dom/DirectionalityUtils.h"
 #include "nsDocument.h"
 #include "nsAttrValueOrString.h"
 #ifdef MOZ_XUL
@@ -128,6 +129,7 @@
 
 using namespace mozilla;
 using namespace mozilla::dom;
+using namespace mozilla::directionality;
 
 nsEventStates
 Element::IntrinsicState() const
@@ -1358,6 +1360,13 @@ nsGenericElement::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
     SetSubtreeRootPointer(aParent->SubtreeRoot());
   }
 
+  // This has to be here, rather than in nsGenericHTMLElement::BindToTree, 
+  //  because it has to happen after updating the parent pointer, but before
+  //  recursively binding the kids.
+  if (IsHTML()) {
+    RecomputeDirectionality(this, false);
+  }
+
   // If NODE_FORCE_XBL_BINDINGS was set we might have anonymous children
   // that also need to be told that they are moving.
   nsresult rv;
@@ -1543,6 +1552,13 @@ nsGenericElement::UnbindFromTree(bool aDeep, bool aNullParent)
     }
   }
 
+  // This has to be here, rather than in nsGenericHTMLElement::UnbindFromTree, 
+  //  because it has to happen after unsetting the parent pointer, but before
+  //  recursively unbinding the kids.
+  if (IsHTML()) {
+    RecomputeDirectionality(this, false);
+  }
+
   if (aDeep) {
     // Do the kids. Don't call GetChildCount() here since that'll force
     // XUL to generate template children, which there is no need for since
diff --git a/content/base/src/nsGkAtomList.h b/content/base/src/nsGkAtomList.h
index e2d75736b662..2049e2847306 100644
--- a/content/base/src/nsGkAtomList.h
+++ b/content/base/src/nsGkAtomList.h
@@ -270,6 +270,7 @@ GK_ATOM(dialog, "dialog")
 GK_ATOM(difference, "difference")
 GK_ATOM(digit, "digit")
 GK_ATOM(dir, "dir")
+GK_ATOM(directionality, "directionality")
 GK_ATOM(disableOutputEscaping, "disable-output-escaping")
 GK_ATOM(disabled, "disabled")
 GK_ATOM(display, "display")
diff --git a/content/html/content/src/nsGenericHTMLElement.cpp b/content/html/content/src/nsGenericHTMLElement.cpp
index 2f502f79a2cd..04ec37904b40 100644
--- a/content/html/content/src/nsGenericHTMLElement.cpp
+++ b/content/html/content/src/nsGenericHTMLElement.cpp
@@ -53,6 +53,7 @@
 
 #include "nsHTMLParts.h"
 #include "nsContentUtils.h"
+#include "mozilla/dom/DirectionalityUtils.h"
 #include "nsString.h"
 #include "nsUnicharUtils.h"
 #include "nsGkAtoms.h"
@@ -96,6 +97,7 @@
 
 using namespace mozilla;
 using namespace mozilla::dom;
+using namespace mozilla::directionality;
 
 class nsINodeInfo;
 class nsIDOMNodeList;
@@ -1687,6 +1689,24 @@ nsGenericHTMLElement::UpdateEditableState(bool aNotify)
   nsStyledElement::UpdateEditableState(aNotify);
 }
 
+nsEventStates
+nsGenericHTMLElement::IntrinsicState() const
+{
+  nsEventStates state = nsGenericHTMLElementBase::IntrinsicState();
+
+  if (GetDirectionality() == eDir_RTL) {
+    state |= NS_EVENT_STATE_RTL;
+    state &= ~NS_EVENT_STATE_LTR;
+  } else { // at least for HTML, directionality is exclusively LTR or RTL
+    NS_ASSERTION(GetDirectionality() == eDir_LTR,
+                 "HTML element's directionality must be either RTL or LTR");
+    state |= NS_EVENT_STATE_LTR;
+    state &= ~NS_EVENT_STATE_RTL;
+  }
+
+  return state;
+}
+
 nsresult
 nsGenericHTMLElement::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
                                  nsIContent* aBindingParent,
@@ -1889,6 +1909,20 @@ nsGenericHTMLElement::AfterSetAttr(PRInt32 aNamespaceID, nsIAtom* aName,
     else if (aNotify && aName == nsGkAtoms::spellcheck) {
       SyncEditorsOnSubtree(this);
     }
+    else if (aName == nsGkAtoms::dir) {
+      Directionality dir;
+      if (aValue &&
+          (aValue->Equals(nsGkAtoms::ltr, eIgnoreCase) ||
+           aValue->Equals(nsGkAtoms::rtl, eIgnoreCase))) {
+        SetHasValidDir();
+        dir = aValue->Equals(nsGkAtoms::rtl, eIgnoreCase) ? eDir_RTL : eDir_LTR;
+        SetDirectionality(dir, aNotify);
+      } else {
+        ClearHasValidDir();
+        dir = RecomputeDirectionality(this, aNotify);
+      }
+      SetDirectionalityOnDescendants(this, dir, aNotify);
+    }
   }
 
   return nsGenericHTMLElementBase::AfterSetAttr(aNamespaceID, aName,
diff --git a/content/html/content/src/nsGenericHTMLElement.h b/content/html/content/src/nsGenericHTMLElement.h
index b1e04ea37824..a4bbd99db54b 100644
--- a/content/html/content/src/nsGenericHTMLElement.h
+++ b/content/html/content/src/nsGenericHTMLElement.h
@@ -50,6 +50,7 @@ public:
     NS_ASSERTION(mNodeInfo->NamespaceID() == kNameSpaceID_XHTML,
                  "Unexpected namespace");
     AddStatesSilently(NS_EVENT_STATE_LTR);
+    SetFlags(NODE_HAS_DIRECTION_LTR);
   }
 
   /** Typesafe, non-refcounting cast from nsIContent.  Cheaper than QI. **/
@@ -203,6 +204,8 @@ public:
 
   virtual void UpdateEditableState(bool aNotify);
 
+  virtual nsEventStates IntrinsicState() const;
+
   // Helper for setting our editable flag and notifying
   void DoSetEditableFlag(bool aEditable, bool aNotify) {
     SetEditableFlag(aEditable);