diff --git a/mozilla/content/base/public/nsINode.h b/mozilla/content/base/public/nsINode.h
index 87f7a3617f3..30f74fcbe0d 100644
--- a/mozilla/content/base/public/nsINode.h
+++ b/mozilla/content/base/public/nsINode.h
@@ -92,8 +92,14 @@ enum {
 
   NODE_IS_EDITABLE =             0x00000100U,
 
+  // Optimizations to quickly check whether element may have ID, class or style
+  // attributes. Not all element implementations may use these!
+  NODE_MAY_HAVE_ID =             0x00000200U,
+  NODE_MAY_HAVE_CLASS =          0x00000400U,
+  NODE_MAY_HAVE_STYLE =          0x00000800U,
+
   // Four bits for the script-type ID
-  NODE_SCRIPT_TYPE_OFFSET =                9,
+  NODE_SCRIPT_TYPE_OFFSET =               12,
 
   NODE_SCRIPT_TYPE_SIZE =                  4,
 
diff --git a/mozilla/content/base/src/nsGenericElement.cpp b/mozilla/content/base/src/nsGenericElement.cpp
index b7f8ec68b04..852d189a400 100644
--- a/mozilla/content/base/src/nsGenericElement.cpp
+++ b/mozilla/content/base/src/nsGenericElement.cpp
@@ -2265,6 +2265,10 @@ nsGenericElement::DispatchDOMEvent(nsEvent* aEvent,
 nsIAtom*
 nsGenericElement::GetID() const
 {
+  if (!HasFlag(NODE_MAY_HAVE_ID)) {
+    return nsnull;
+  }
+
   nsIAtom* IDName = GetIDAttributeName();
   if (IDName) {
     const nsAttrValue* attrVal = mAttrsAndChildren.GetAttr(IDName);
@@ -2287,6 +2291,9 @@ nsGenericElement::GetID() const
       }
     }
   }
+
+  nsINode* node = const_cast<nsINode*>(static_cast<const nsINode*>(this));
+  node->UnsetFlags(NODE_MAY_HAVE_ID);
   return nsnull;
 }
 
@@ -3680,6 +3687,7 @@ nsGenericElement::ParseAttribute(PRInt32 aNamespaceID,
 {
   if (aNamespaceID == kNameSpaceID_None &&
       aAttribute == GetIDAttributeName() && !aValue.IsEmpty()) {
+    SetFlags(NODE_MAY_HAVE_ID);
     // Store id as an atom.  id="" means that the element has no id,
     // not that it has an emptystring as the id.
     aResult.ParseAtom(aValue);
diff --git a/mozilla/content/base/src/nsStyledElement.cpp b/mozilla/content/base/src/nsStyledElement.cpp
index 454fdbf7c9f..0967391c4f7 100644
--- a/mozilla/content/base/src/nsStyledElement.cpp
+++ b/mozilla/content/base/src/nsStyledElement.cpp
@@ -73,7 +73,15 @@ nsStyledElement::GetIDAttributeName() const
 const nsAttrValue*
 nsStyledElement::GetClasses() const
 {
-  return mAttrsAndChildren.GetAttr(nsGkAtoms::_class);
+  if (!HasFlag(NODE_MAY_HAVE_CLASS)) {
+    return nsnull;
+  }
+  const nsAttrValue* clazz = mAttrsAndChildren.GetAttr(nsGkAtoms::_class);
+  if (!clazz) {
+    nsINode* node = const_cast<nsINode*>(static_cast<const nsINode*>(this));
+    node->UnsetFlags(NODE_MAY_HAVE_CLASS);
+  }
+  return clazz;
 }
 
 PRBool
@@ -82,10 +90,12 @@ nsStyledElement::ParseAttribute(PRInt32 aNamespaceID, nsIAtom* aAttribute,
 {
   if (aNamespaceID == kNameSpaceID_None) {
     if (aAttribute == nsGkAtoms::style) {
+      SetFlags(NODE_MAY_HAVE_STYLE);
       ParseStyleAttribute(this, aValue, aResult);
       return PR_TRUE;
     }
     if (aAttribute == nsGkAtoms::_class) {
+      SetFlags(NODE_MAY_HAVE_CLASS);
 #ifdef MOZ_SVG
       NS_ASSERTION(!nsCOMPtr<nsIDOMSVGStylable>(do_QueryInterface(this)),
                    "SVG code should have handled this 'class' attribute!");
@@ -102,6 +112,7 @@ nsStyledElement::ParseAttribute(PRInt32 aNamespaceID, nsIAtom* aAttribute,
 NS_IMETHODIMP
 nsStyledElement::SetInlineStyleRule(nsICSSStyleRule* aStyleRule, PRBool aNotify)
 {
+  SetFlags(NODE_MAY_HAVE_STYLE);
   PRBool modification = PR_FALSE;
   nsAutoString oldValueStr;
 
@@ -135,12 +146,16 @@ nsStyledElement::SetInlineStyleRule(nsICSSStyleRule* aStyleRule, PRBool aNotify)
 nsICSSStyleRule*
 nsStyledElement::GetInlineStyleRule()
 {
+  if (!HasFlag(NODE_MAY_HAVE_STYLE)) {
+    return nsnull;
+  }
   const nsAttrValue* attrVal = mAttrsAndChildren.GetAttr(nsGkAtoms::style);
 
   if (attrVal && attrVal->Type() == nsAttrValue::eCSSStyleRule) {
     return attrVal->GetCSSStyleRuleValue();
   }
 
+  UnsetFlags(NODE_MAY_HAVE_STYLE);
   return nsnull;
 }
 
@@ -196,6 +211,9 @@ nsStyledElement::GetStyle(nsIDOMCSSStyleDeclaration** aStyle)
 nsresult
 nsStyledElement::ReparseStyleAttribute()
 {
+  if (!HasFlag(NODE_MAY_HAVE_STYLE)) {
+    return NS_OK;
+  }
   const nsAttrValue* oldVal = mAttrsAndChildren.GetAttr(nsGkAtoms::style);
   
   if (oldVal && oldVal->Type() != nsAttrValue::eCSSStyleRule) {