diff --git a/components/layout/construct.rs b/components/layout/construct.rs
index dd1d58a6acce..8b4fd32d56c2 100644
--- a/components/layout/construct.rs
+++ b/components/layout/construct.rs
@@ -1430,6 +1430,8 @@ impl<'a> PostorderNodeMutTraversal for FlowConstructor<'a> {
             Some(NodeTypeId::Document) => {
                 (display::T::none, float::T::none, position::T::static_)
             }
+            Some(NodeTypeId::Node) => unreachable!(),
+            Some(NodeTypeId::CharacterData(CharacterDataTypeId::CharacterData)) => unreachable!(),
         };
 
         debug!("building flow for node: {:?} {:?} {:?} {:?}", display, float, positioning, node.type_id());
@@ -1585,6 +1587,7 @@ impl<'ln> NodeUtils for ThreadSafeLayoutNode<'ln> {
             Some(NodeTypeId::Element(ElementTypeId::HTMLElement(
                         HTMLElementTypeId::HTMLObjectElement))) => self.has_object_data(),
             Some(NodeTypeId::Element(_)) => false,
+            Some(NodeTypeId::Node) => unreachable!(),
         }
     }
 
diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index 5b3a3de2f186..d0b45e350a3f 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -1722,6 +1722,25 @@ def build(namespaces, child, public=False):
         return CGNamespace(namespaces[0], inner, public=public)
 
 
+def EventTargetEnum(desc):
+    protochain = desc.prototypeChain
+    if protochain[0] != "EventTarget":
+        return "None"
+
+    inner = ""
+    name = desc.interface.identifier.name
+    if desc.interface.getUserData("hasConcreteDescendant", False):
+        inner = "(::dom::%s::%sTypeId::%s)" % (name.lower(), name, name)
+    prev_proto = ""
+    for proto in reversed(protochain):
+        if prev_proto != "":
+            inner = "(::dom::%s::%sTypeId::%s%s)" % (proto.lower(), proto, prev_proto, inner)
+        prev_proto = proto
+    if inner == "":
+        return "None"
+    return "Some%s" % inner
+
+
 def DOMClass(descriptor):
         protoList = ['PrototypeList::ID::' + proto for proto in descriptor.prototypeChain]
         # Pad out the list to the right length with ID::Count so we
@@ -1734,7 +1753,8 @@ def DOMClass(descriptor):
 DOMClass {
     interface_chain: [ %s ],
     native_hooks: &sNativePropertyHooks,
-}""" % prototypeChainString
+    type_id: %s,
+}""" % (prototypeChainString, EventTargetEnum(descriptor))
 
 
 class CGDOMJSClass(CGThing):
diff --git a/components/script/dom/bindings/codegen/Configuration.py b/components/script/dom/bindings/codegen/Configuration.py
index 1a88968619bf..2573e96fc785 100644
--- a/components/script/dom/bindings/codegen/Configuration.py
+++ b/components/script/dom/bindings/codegen/Configuration.py
@@ -223,8 +223,9 @@ def addIndexedOrNamedOperation(operation, m):
                     if m.isDeleter():
                         addIndexedOrNamedOperation('Deleter', m)
 
-                iface.setUserData('hasConcreteDescendant', True)
                 iface = iface.parent
+                if iface:
+                    iface.setUserData('hasConcreteDescendant', True)
 
             if self.proxy:
                 iface = self.interface
diff --git a/components/script/dom/bindings/conversions.rs b/components/script/dom/bindings/conversions.rs
index 35f7be92b948..7118bef0e685 100644
--- a/components/script/dom/bindings/conversions.rs
+++ b/components/script/dom/bindings/conversions.rs
@@ -645,7 +645,7 @@ pub unsafe fn native_from_reflector<T>(obj: *mut JSObject) -> *const T {
 }
 
 /// Get the `DOMClass` from `obj`, or `Err(())` if `obj` is not a DOM object.
-unsafe fn get_dom_class(obj: *mut JSObject) -> Result<DOMClass, ()> {
+pub unsafe fn get_dom_class(obj: *mut JSObject) -> Result<&'static DOMClass, ()> {
     use dom::bindings::utils::DOMJSClass;
     use js::glue::GetProxyHandlerExtra;
 
@@ -653,12 +653,12 @@ unsafe fn get_dom_class(obj: *mut JSObject) -> Result<DOMClass, ()> {
     if is_dom_class(&*clasp) {
         debug!("plain old dom object");
         let domjsclass: *const DOMJSClass = clasp as *const DOMJSClass;
-        return Ok((*domjsclass).dom_class);
+        return Ok(&(&*domjsclass).dom_class);
     }
     if is_dom_proxy(obj) {
         debug!("proxy dom object");
         let dom_class: *const DOMClass = GetProxyHandlerExtra(obj) as *const DOMClass;
-        return Ok(*dom_class);
+        return Ok(&*dom_class);
     }
     debug!("not a dom object");
     Err(())
diff --git a/components/script/dom/bindings/utils.rs b/components/script/dom/bindings/utils.rs
index 08e8fde72250..be600d807d37 100644
--- a/components/script/dom/bindings/utils.rs
+++ b/components/script/dom/bindings/utils.rs
@@ -15,6 +15,7 @@ use dom::bindings::global::GlobalRef;
 use dom::bindings::js::Root;
 use dom::bindings::trace::trace_object;
 use dom::browsercontext;
+use dom::eventtarget::EventTargetTypeId;
 use dom::window;
 use util::mem::HeapSizeOf;
 use util::str::DOMString;
@@ -157,6 +158,9 @@ pub struct DOMClass {
     /// derivedness.
     pub interface_chain: [PrototypeList::ID; MAX_PROTO_CHAIN_LENGTH],
 
+    /// The EventTarget type, if this is derived from an EventTarget.
+    pub type_id: Option<EventTargetTypeId>,
+
     /// The NativePropertyHooks for the interface associated with this class.
     pub native_hooks: &'static NativePropertyHooks,
 }
diff --git a/components/script/dom/characterdata.rs b/components/script/dom/characterdata.rs
index 3efcd7c16c28..d4a2c5e04784 100644
--- a/components/script/dom/characterdata.rs
+++ b/components/script/dom/characterdata.rs
@@ -158,8 +158,10 @@ impl CharacterDataMethods for CharacterData {
 }
 
 /// The different types of CharacterData.
-#[derive(JSTraceable, Copy, Clone, PartialEq, Debug, HeapSizeOf)]
+#[derive(Copy, Clone, PartialEq, Debug)]
 pub enum CharacterDataTypeId {
+    CharacterData,
+
     Comment,
     Text,
     ProcessingInstruction,
diff --git a/components/script/dom/dedicatedworkerglobalscope.rs b/components/script/dom/dedicatedworkerglobalscope.rs
index a3c033e56a69..9ced72210783 100644
--- a/components/script/dom/dedicatedworkerglobalscope.rs
+++ b/components/script/dom/dedicatedworkerglobalscope.rs
@@ -163,8 +163,7 @@ impl DedicatedWorkerGlobalScope {
                      -> DedicatedWorkerGlobalScope {
         DedicatedWorkerGlobalScope {
             workerglobalscope: WorkerGlobalScope::new_inherited(
-                WorkerGlobalScopeTypeId::DedicatedGlobalScope, init, worker_url,
-                runtime, from_devtools_receiver),
+                init, worker_url, runtime, from_devtools_receiver),
             id: id,
             receiver: receiver,
             own_sender: own_sender,
@@ -364,7 +363,7 @@ impl DedicatedWorkerGlobalScopeMethods for DedicatedWorkerGlobalScope {
 impl DedicatedWorkerGlobalScopeDerived for EventTarget {
     fn is_dedicatedworkerglobalscope(&self) -> bool {
         match *self.type_id() {
-            EventTargetTypeId::WorkerGlobalScope(WorkerGlobalScopeTypeId::DedicatedGlobalScope) => true,
+            EventTargetTypeId::WorkerGlobalScope(WorkerGlobalScopeTypeId::DedicatedWorkerGlobalScope) => true,
             _ => false
         }
     }
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 27e8e9c7b7ec..bd522dccf0af 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -116,7 +116,7 @@ impl PartialEq for Element {
     }
 }
 
-#[derive(JSTraceable, Copy, Clone, PartialEq, Debug, HeapSizeOf)]
+#[derive(Copy, Clone, PartialEq, Debug)]
 pub enum ElementTypeId {
     HTMLElement(HTMLElementTypeId),
     Element,
diff --git a/components/script/dom/eventtarget.rs b/components/script/dom/eventtarget.rs
index f2f228f2c0df..6ac33876e09d 100644
--- a/components/script/dom/eventtarget.rs
+++ b/components/script/dom/eventtarget.rs
@@ -7,6 +7,7 @@ use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::EventHandlerBinding::EventHandlerNonNull;
 use dom::bindings::codegen::Bindings::EventListenerBinding::EventListener;
 use dom::bindings::codegen::Bindings::EventTargetBinding::EventTargetMethods;
+use dom::bindings::conversions::get_dom_class;
 use dom::bindings::error::Error::InvalidState;
 use dom::bindings::error::{Fallible, report_pending_exception};
 use dom::bindings::utils::{Reflectable, Reflector};
@@ -45,9 +46,10 @@ pub enum ListenerPhase {
     Bubbling,
 }
 
-#[derive(JSTraceable, Copy, Clone)]
-#[derive(HeapSizeOf)]
+#[derive(Copy, Clone)]
 pub enum EventTargetTypeId {
+    EventTarget,
+
     Node(NodeTypeId),
     WebSocket,
     Window,
@@ -132,15 +134,13 @@ pub struct EventListenerEntry {
 #[dom_struct]
 pub struct EventTarget {
     reflector_: Reflector,
-    type_id: EventTargetTypeId,
     handlers: DOMRefCell<HashMap<DOMString, Vec<EventListenerEntry>, DefaultState<FnvHasher>>>,
 }
 
 impl EventTarget {
-    pub fn new_inherited(type_id: EventTargetTypeId) -> EventTarget {
+    pub fn new_inherited() -> EventTarget {
         EventTarget {
             reflector_: Reflector::new(),
-            type_id: type_id,
             handlers: DOMRefCell::new(Default::default()),
         }
     }
@@ -159,9 +159,12 @@ impl EventTarget {
         })
     }
 
-    #[inline]
+    #[allow(unsafe_code)]
     pub fn type_id(&self) -> &EventTargetTypeId {
-        &self.type_id
+        let domclass = unsafe {
+            get_dom_class(self.reflector_.get_jsobject().get()).unwrap()
+        };
+        domclass.type_id.as_ref().unwrap()
     }
 
     pub fn dispatch_event_with_target(&self,
diff --git a/components/script/dom/filereader.rs b/components/script/dom/filereader.rs
index ffd74806c04d..5de6f4dd5f99 100644
--- a/components/script/dom/filereader.rs
+++ b/components/script/dom/filereader.rs
@@ -15,7 +15,7 @@ use dom::bindings::utils::{reflect_dom_object, Reflectable};
 use dom::blob::Blob;
 use dom::domexception::{DOMException, DOMErrorName};
 use dom::event::{EventCancelable, EventBubbles};
-use dom::eventtarget::{EventTarget, EventTargetTypeId};
+use dom::eventtarget::EventTarget;
 use dom::progressevent::ProgressEvent;
 use encoding::all::UTF_8;
 use encoding::label::encoding_from_whatwg_label;
@@ -80,7 +80,7 @@ pub struct FileReader {
 impl FileReader {
     pub fn new_inherited(global: GlobalRef) -> FileReader {
         FileReader {
-            eventtarget: EventTarget::new_inherited(EventTargetTypeId::FileReader),//?
+            eventtarget: EventTarget::new_inherited(),//?
             global: GlobalField::from_rooted(&global),
             ready_state: Cell::new(FileReaderReadyState::Empty),
             error: MutNullableHeap::new(None),
diff --git a/components/script/dom/htmlelement.rs b/components/script/dom/htmlelement.rs
index 661378a7fc40..9a02a47dd0f8 100644
--- a/components/script/dom/htmlelement.rs
+++ b/components/script/dom/htmlelement.rs
@@ -344,7 +344,7 @@ impl VirtualMethods for HTMLElement {
     }
 }
 
-#[derive(JSTraceable, Copy, Clone, Debug, HeapSizeOf)]
+#[derive(Copy, Clone, Debug)]
 pub enum HTMLElementTypeId {
     HTMLElement,
 
diff --git a/components/script/dom/htmlmediaelement.rs b/components/script/dom/htmlmediaelement.rs
index 4b40e13e1292..7a2b798c7285 100644
--- a/components/script/dom/htmlmediaelement.rs
+++ b/components/script/dom/htmlmediaelement.rs
@@ -41,8 +41,10 @@ impl HTMLMediaElement {
     }
 }
 
-#[derive(JSTraceable, Copy, Clone, Debug, HeapSizeOf)]
+#[derive(Copy, Clone, Debug)]
 pub enum HTMLMediaElementTypeId {
+    HTMLMediaElement = -1,
+
     HTMLAudioElement = 0,
     HTMLVideoElement = 1,
 }
diff --git a/components/script/dom/htmltablecellelement.rs b/components/script/dom/htmltablecellelement.rs
index c67d02c86937..e1f8179a19b7 100644
--- a/components/script/dom/htmltablecellelement.rs
+++ b/components/script/dom/htmltablecellelement.rs
@@ -22,8 +22,10 @@ use std::cmp::max;
 
 const DEFAULT_COLSPAN: u32 = 1;
 
-#[derive(JSTraceable, Copy, Clone, Debug, HeapSizeOf)]
+#[derive(Copy, Clone, Debug)]
 pub enum HTMLTableCellElementTypeId {
+    HTMLTableCellElement = -1,
+
     HTMLTableDataCellElement = 0,
     HTMLTableHeaderCellElement = 1,
 }
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index b0c8d79228eb..5f437fe0cad7 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -81,9 +81,6 @@ pub struct Node {
     /// The JavaScript reflector for this node.
     eventtarget: EventTarget,
 
-    /// The type of node that this is.
-    type_id: NodeTypeId,
-
     /// The parent of this node.
     parent_node: MutNullableHeap<JS<Node>>,
 
@@ -284,9 +281,10 @@ impl LayoutDataRef {
 }
 
 /// The different types of nodes.
-#[derive(JSTraceable, Copy, Clone, PartialEq, Debug)]
-#[derive(HeapSizeOf)]
+#[derive(Copy, Clone, PartialEq, Debug)]
 pub enum NodeTypeId {
+    Node,
+
     CharacterData(CharacterDataTypeId),
     DocumentType,
     DocumentFragment,
@@ -452,7 +450,7 @@ impl Node {
 
     /// Returns a string that describes this node.
     pub fn debug_str(&self) -> String {
-        format!("{:?}", self.type_id)
+        format!("{:?}", self.type_id())
     }
 
     pub fn is_in_doc(&self) -> bool {
@@ -461,12 +459,15 @@ impl Node {
 
     /// Returns the type ID of this node. Fails if this node is borrowed mutably.
     pub fn type_id(&self) -> NodeTypeId {
-        self.type_id
+        match *self.eventtarget.type_id() {
+            EventTargetTypeId::Node(type_id) => type_id,
+            _ => unreachable!(),
+        }
     }
 
     // https://dom.spec.whatwg.org/#concept-node-length
     pub fn len(&self) -> u32 {
-        match self.type_id {
+        match self.type_id() {
             NodeTypeId::DocumentType => 0,
             NodeTypeId::CharacterData(_) => {
                 CharacterDataCast::to_ref(self).unwrap().Length()
@@ -486,12 +487,12 @@ impl Node {
 
     #[inline]
     pub fn is_anchor_element(&self) -> bool {
-        self.type_id == NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLAnchorElement))
+        self.type_id() == NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLAnchorElement))
     }
 
     #[inline]
     pub fn is_doctype(&self) -> bool {
-        self.type_id == NodeTypeId::DocumentType
+        self.type_id() == NodeTypeId::DocumentType
     }
 
     pub fn get_flag(&self, flag: NodeFlags) -> bool {
@@ -1026,7 +1027,7 @@ impl LayoutNodeHelpers for LayoutJS<Node> {
     #[inline]
     #[allow(unsafe_code)]
     unsafe fn type_id_for_layout(&self) -> NodeTypeId {
-        (*self.unsafe_get()).type_id
+        (*self.unsafe_get()).type_id()
     }
 
     #[inline]
@@ -1393,8 +1394,7 @@ impl Node {
 
     fn new_(type_id: NodeTypeId, doc: Option<&Document>) -> Node {
         Node {
-            eventtarget: EventTarget::new_inherited(EventTargetTypeId::Node(type_id)),
-            type_id: type_id,
+            eventtarget: EventTarget::new_inherited(),
 
             parent_node: Default::default(),
             first_child: Default::default(),
@@ -1456,6 +1456,8 @@ impl Node {
 
         // Step 4-5.
         match node.type_id() {
+            NodeTypeId::Node => unreachable!(),
+            NodeTypeId::CharacterData(CharacterDataTypeId::CharacterData) => unreachable!(),
             NodeTypeId::CharacterData(CharacterDataTypeId::Text) => {
                 if parent.is_document() {
                     return Err(HierarchyRequest);
@@ -1538,7 +1540,7 @@ impl Node {
                     }
                 },
                 NodeTypeId::CharacterData(_) => (),
-                NodeTypeId::Document => unreachable!(),
+                NodeTypeId::Document | NodeTypeId::Node => unreachable!(),
             }
         }
         Ok(())
@@ -1698,6 +1700,8 @@ impl Node {
         // Step 2.
         // XXXabinader: clone() for each node as trait?
         let copy: Root<Node> = match node.type_id() {
+            NodeTypeId::Node => unreachable!(),
+            NodeTypeId::CharacterData(CharacterDataTypeId::CharacterData) => unreachable!(),
             NodeTypeId::DocumentType => {
                 let doctype: &DocumentType = DocumentTypeCast::to_ref(node).unwrap();
                 let doctype = DocumentType::new(doctype.name().clone(),
@@ -1833,7 +1837,7 @@ impl Node {
                 }
         }
 
-        match node.type_id {
+        match node.type_id() {
             NodeTypeId::Element(_) => {
                 let element = ElementCast::to_ref(node).unwrap();
                 // Step 1.
@@ -1889,7 +1893,9 @@ impl Node {
 impl NodeMethods for Node {
     // https://dom.spec.whatwg.org/#dom-node-nodetype
     fn NodeType(&self) -> u16 {
-        match self.type_id {
+        match self.type_id() {
+            NodeTypeId::Node => unreachable!(),
+            NodeTypeId::CharacterData(CharacterDataTypeId::CharacterData) => unreachable!(),
             NodeTypeId::CharacterData(CharacterDataTypeId::Text) =>
                 NodeConstants::TEXT_NODE,
             NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction) =>
@@ -1909,11 +1915,13 @@ impl NodeMethods for Node {
 
     // https://dom.spec.whatwg.org/#dom-node-nodename
     fn NodeName(&self) -> DOMString {
-        match self.type_id {
+        match self.type_id() {
+            NodeTypeId::Node => unreachable!(),
             NodeTypeId::Element(..) => {
                 let elem: &Element = ElementCast::to_ref(self).unwrap();
                 elem.TagName()
             }
+            NodeTypeId::CharacterData(CharacterDataTypeId::CharacterData) => unreachable!(),
             NodeTypeId::CharacterData(CharacterDataTypeId::Text) => "#text".to_owned(),
             NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction) => {
                 let processing_instruction: &ProcessingInstruction =
@@ -1937,7 +1945,8 @@ impl NodeMethods for Node {
 
     // https://dom.spec.whatwg.org/#dom-node-ownerdocument
     fn GetOwnerDocument(&self) -> Option<Root<Document>> {
-        match self.type_id {
+        match self.type_id() {
+            NodeTypeId::Node => unreachable!(),
             NodeTypeId::CharacterData(..) |
             NodeTypeId::Element(..) |
             NodeTypeId::DocumentType |
@@ -1992,19 +2001,31 @@ impl NodeMethods for Node {
 
     // https://dom.spec.whatwg.org/#dom-node-nodevalue
     fn GetNodeValue(&self) -> Option<DOMString> {
-        CharacterDataCast::to_ref(self).map(|c| c.Data())
+        match self.type_id() {
+            NodeTypeId::CharacterData(..) => {
+                let chardata: &CharacterData = CharacterDataCast::to_ref(self).unwrap();
+                Some(chardata.Data())
+            }
+            _ => {
+                None
+            }
+        }
     }
 
     // https://dom.spec.whatwg.org/#dom-node-nodevalue
     fn SetNodeValue(&self, val: Option<DOMString>) {
-        if let NodeTypeId::CharacterData(..) = self.type_id {
-            self.SetTextContent(val)
+        match self.type_id() {
+            NodeTypeId::CharacterData(..) => {
+                self.SetTextContent(val)
+            }
+            _ => {}
         }
     }
 
     // https://dom.spec.whatwg.org/#dom-node-textcontent
     fn GetTextContent(&self) -> Option<DOMString> {
-        match self.type_id {
+        match self.type_id() {
+            NodeTypeId::Node => unreachable!(),
             NodeTypeId::DocumentFragment |
             NodeTypeId::Element(..) => {
                 let content = Node::collect_text_contents(self.traverse_preorder());
@@ -2024,7 +2045,8 @@ impl NodeMethods for Node {
     // https://dom.spec.whatwg.org/#dom-node-textcontent
     fn SetTextContent(&self, value: Option<DOMString>) {
         let value = value.unwrap_or(String::new());
-        match self.type_id {
+        match self.type_id() {
+            NodeTypeId::Node => unreachable!(),
             NodeTypeId::DocumentFragment |
             NodeTypeId::Element(..) => {
                 // Step 1-2.
@@ -2065,7 +2087,7 @@ impl NodeMethods for Node {
     fn ReplaceChild(&self, node: &Node, child: &Node) -> Fallible<Root<Node>> {
 
         // Step 1.
-        match self.type_id {
+        match self.type_id() {
             NodeTypeId::Document |
             NodeTypeId::DocumentFragment |
             NodeTypeId::Element(..) => (),
@@ -2084,6 +2106,7 @@ impl NodeMethods for Node {
 
         // Step 4-5.
         match node.type_id() {
+            NodeTypeId::Node => unreachable!(),
             NodeTypeId::CharacterData(CharacterDataTypeId::Text) if self.is_document() =>
                 return Err(HierarchyRequest),
             NodeTypeId::DocumentType if !self.is_document() => return Err(HierarchyRequest),
@@ -2147,7 +2170,7 @@ impl NodeMethods for Node {
                     }
                 },
                 NodeTypeId::CharacterData(..) => (),
-                NodeTypeId::Document => unreachable!()
+                NodeTypeId::Document | NodeTypeId::Node => unreachable!()
             }
         }
 
diff --git a/components/script/dom/websocket.rs b/components/script/dom/websocket.rs
index 4b614786efa9..2bf84d609e71 100644
--- a/components/script/dom/websocket.rs
+++ b/components/script/dom/websocket.rs
@@ -20,7 +20,7 @@ use dom::bindings::utils::{reflect_dom_object, Reflectable};
 use dom::blob::Blob;
 use dom::closeevent::CloseEvent;
 use dom::event::{Event, EventBubbles, EventCancelable};
-use dom::eventtarget::{EventTarget, EventTargetTypeId};
+use dom::eventtarget::EventTarget;
 use dom::messageevent::MessageEvent;
 use script_task::ScriptTaskEventCategory::WebSocketEvent;
 use script_task::{Runnable, CommonScriptMsg};
@@ -110,7 +110,7 @@ fn establish_a_websocket_connection(resource_url: &Url, net_url: (Host, String,
 impl WebSocket {
     fn new_inherited(global: GlobalRef, url: Url) -> WebSocket {
         WebSocket {
-            eventtarget: EventTarget::new_inherited(EventTargetTypeId::WebSocket),
+            eventtarget: EventTarget::new_inherited(),
             url: url,
             global: GlobalField::from_rooted(&global),
             ready_state: Cell::new(WebSocketRequestState::Connecting),
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 2ca420d441d3..484685584c2a 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -1288,7 +1288,7 @@ impl Window {
         };
 
         let win = box Window {
-            eventtarget: EventTarget::new_inherited(EventTargetTypeId::Window),
+            eventtarget: EventTarget::new_inherited(),
             script_chan: script_chan,
             image_cache_chan: image_cache_chan,
             control_chan: control_chan,
diff --git a/components/script/dom/worker.rs b/components/script/dom/worker.rs
index 2711e8502db5..92053392db76 100644
--- a/components/script/dom/worker.rs
+++ b/components/script/dom/worker.rs
@@ -17,7 +17,7 @@ use dom::bindings::utils::{Reflectable, reflect_dom_object};
 use dom::dedicatedworkerglobalscope::{DedicatedWorkerGlobalScope, WorkerScriptMsg};
 use dom::errorevent::ErrorEvent;
 use dom::event::{Event, EventBubbles, EventCancelable};
-use dom::eventtarget::{EventTarget, EventTargetTypeId};
+use dom::eventtarget::EventTarget;
 use dom::messageevent::MessageEvent;
 use dom::workerglobalscope::WorkerGlobalScopeInit;
 
@@ -52,7 +52,7 @@ impl Worker {
                      sender: Sender<(TrustedWorkerAddress, WorkerScriptMsg)>)
                      -> Worker {
         Worker {
-            eventtarget: EventTarget::new_inherited(EventTargetTypeId::Worker),
+            eventtarget: EventTarget::new_inherited(),
             global: GlobalField::from_rooted(&global),
             sender: sender,
         }
diff --git a/components/script/dom/workerglobalscope.rs b/components/script/dom/workerglobalscope.rs
index 3bc805a67e67..5d5e944ccf23 100644
--- a/components/script/dom/workerglobalscope.rs
+++ b/components/script/dom/workerglobalscope.rs
@@ -12,7 +12,7 @@ use dom::bindings::js::{JS, Root, MutNullableHeap};
 use dom::bindings::utils::Reflectable;
 use dom::console::Console;
 use dom::crypto::Crypto;
-use dom::eventtarget::{EventTarget, EventTargetTypeId};
+use dom::eventtarget::EventTarget;
 use dom::window::{base64_atob, base64_btoa};
 use dom::workerlocation::WorkerLocation;
 use dom::workernavigator::WorkerNavigator;
@@ -36,9 +36,11 @@ use std::default::Default;
 use std::rc::Rc;
 use std::sync::mpsc::Receiver;
 
-#[derive(JSTraceable, Copy, Clone, PartialEq, HeapSizeOf)]
+#[derive(Copy, Clone, PartialEq)]
 pub enum WorkerGlobalScopeTypeId {
-    DedicatedGlobalScope,
+    WorkerGlobalScope,
+
+    DedicatedWorkerGlobalScope,
 }
 
 pub struct WorkerGlobalScopeInit {
@@ -90,14 +92,13 @@ pub struct WorkerGlobalScope {
 }
 
 impl WorkerGlobalScope {
-    pub fn new_inherited(type_id: WorkerGlobalScopeTypeId,
-                         init: WorkerGlobalScopeInit,
+    pub fn new_inherited(init: WorkerGlobalScopeInit,
                          worker_url: Url,
                          runtime: Rc<Runtime>,
                          from_devtools_receiver: Receiver<DevtoolScriptControlMsg>)
                          -> WorkerGlobalScope {
         WorkerGlobalScope {
-            eventtarget: EventTarget::new_inherited(EventTargetTypeId::WorkerGlobalScope(type_id)),
+            eventtarget: EventTarget::new_inherited(),
             next_worker_id: Cell::new(WorkerId(0)),
             worker_id: init.worker_id,
             worker_url: worker_url,
diff --git a/components/script/dom/xmlhttprequest.rs b/components/script/dom/xmlhttprequest.rs
index 4dd5647029a0..743c0e2f6dd7 100644
--- a/components/script/dom/xmlhttprequest.rs
+++ b/components/script/dom/xmlhttprequest.rs
@@ -155,7 +155,7 @@ pub struct XMLHttpRequest {
 impl XMLHttpRequest {
     fn new_inherited(global: GlobalRef) -> XMLHttpRequest {
         XMLHttpRequest {
-            eventtarget: XMLHttpRequestEventTarget::new_inherited(XMLHttpRequestEventTargetTypeId::XMLHttpRequest),
+            eventtarget: XMLHttpRequestEventTarget::new_inherited(),
             ready_state: Cell::new(XMLHttpRequestState::Unsent),
             timeout: Cell::new(0u32),
             with_credentials: Cell::new(false),
diff --git a/components/script/dom/xmlhttprequesteventtarget.rs b/components/script/dom/xmlhttprequesteventtarget.rs
index 627e4882dded..20452261f949 100644
--- a/components/script/dom/xmlhttprequesteventtarget.rs
+++ b/components/script/dom/xmlhttprequesteventtarget.rs
@@ -8,8 +8,10 @@ use dom::bindings::codegen::InheritTypes::EventTargetCast;
 use dom::bindings::codegen::InheritTypes::XMLHttpRequestEventTargetDerived;
 use dom::eventtarget::{EventTarget, EventTargetTypeId};
 
-#[derive(JSTraceable, Copy, Clone, PartialEq, HeapSizeOf)]
+#[derive(Copy, Clone, PartialEq)]
 pub enum XMLHttpRequestEventTargetTypeId {
+    XMLHttpRequestEventTarget,
+
     XMLHttpRequest,
     XMLHttpRequestUpload,
 }
@@ -20,9 +22,9 @@ pub struct XMLHttpRequestEventTarget {
 }
 
 impl XMLHttpRequestEventTarget {
-    pub fn new_inherited(type_id: XMLHttpRequestEventTargetTypeId) -> XMLHttpRequestEventTarget {
+    pub fn new_inherited() -> XMLHttpRequestEventTarget {
         XMLHttpRequestEventTarget {
-            eventtarget: EventTarget::new_inherited(EventTargetTypeId::XMLHttpRequestEventTarget(type_id))
+            eventtarget: EventTarget::new_inherited()
         }
     }
 }
diff --git a/components/script/dom/xmlhttprequestupload.rs b/components/script/dom/xmlhttprequestupload.rs
index d112d9d1e08b..c7d4d4d4ca30 100644
--- a/components/script/dom/xmlhttprequestupload.rs
+++ b/components/script/dom/xmlhttprequestupload.rs
@@ -19,8 +19,7 @@ pub struct XMLHttpRequestUpload {
 impl XMLHttpRequestUpload {
     fn new_inherited() -> XMLHttpRequestUpload {
         XMLHttpRequestUpload {
-            eventtarget: XMLHttpRequestEventTarget::new_inherited(
-                XMLHttpRequestEventTargetTypeId::XMLHttpRequestUpload)
+            eventtarget: XMLHttpRequestEventTarget::new_inherited(),
         }
     }
     pub fn new(global: GlobalRef) -> Root<XMLHttpRequestUpload> {
diff --git a/components/script/mem.rs b/components/script/mem.rs
index b3a236866bbf..55c92f6e4e4d 100644
--- a/components/script/mem.rs
+++ b/components/script/mem.rs
@@ -8,8 +8,12 @@ use dom::bindings::codegen::InheritTypes::*;
 use dom::element::ElementTypeId;
 use dom::eventtarget::{EventTarget, EventTargetTypeId};
 use dom::htmlelement::HTMLElementTypeId;
-use dom::htmlmediaelement::HTMLMediaElementTypeId::{HTMLAudioElement, HTMLVideoElement};
-use dom::htmltablecellelement::HTMLTableCellElementTypeId::{HTMLTableDataCellElement, HTMLTableHeaderCellElement};
+use dom::htmlmediaelement::HTMLMediaElementTypeId::HTMLAudioElement;
+use dom::htmlmediaelement::HTMLMediaElementTypeId::HTMLMediaElement;
+use dom::htmlmediaelement::HTMLMediaElementTypeId::HTMLVideoElement;
+use dom::htmltablecellelement::HTMLTableCellElementTypeId::HTMLTableCellElement;
+use dom::htmltablecellelement::HTMLTableCellElementTypeId::HTMLTableDataCellElement;
+use dom::htmltablecellelement::HTMLTableCellElementTypeId::HTMLTableHeaderCellElement;
 use dom::node::NodeTypeId;
 use libc;
 use util::mem::{HeapSizeOf, heap_size_of};
@@ -245,5 +249,13 @@ pub fn heap_size_of_eventtarget(target: &EventTarget) -> usize {
             heap_size_of_self_and_children(DocumentTypeCast::to_ref(target).unwrap()),
         &EventTargetTypeId::Node(NodeTypeId::DocumentFragment) =>
             heap_size_of_self_and_children(DocumentFragmentCast::to_ref(target).unwrap()),
+        &EventTargetTypeId::EventTarget |
+        &EventTargetTypeId::Node(NodeTypeId::Node) |
+        &EventTargetTypeId::Node(NodeTypeId::Element(ElementTypeId::HTMLElement(
+                HTMLElementTypeId::HTMLMediaElement(HTMLMediaElement)))) |
+        &EventTargetTypeId::Node(NodeTypeId::Element(ElementTypeId::HTMLElement(
+                HTMLElementTypeId::HTMLTableCellElement(HTMLTableCellElement)))) => {
+            unreachable!()
+        },
     }
 }
diff --git a/components/script/parse/html.rs b/components/script/parse/html.rs
index 8aec2e816e4b..250b128e5f33 100644
--- a/components/script/parse/html.rs
+++ b/components/script/parse/html.rs
@@ -184,6 +184,8 @@ impl<'a> Serializable for &'a Node {
                                  traversal_scope: TraversalScope) -> io::Result<()> {
         let node = *self;
         match (traversal_scope, node.type_id()) {
+            (_, NodeTypeId::Node) => unreachable!(),
+            (_, NodeTypeId::CharacterData(CharacterDataTypeId::CharacterData)) => unreachable!(),
             (_, NodeTypeId::Element(..)) => {
                 let elem = ElementCast::to_ref(node).unwrap();
                 let name = QualName::new(elem.namespace().clone(),
diff --git a/tests/unit/script/size_of.rs b/tests/unit/script/size_of.rs
index cea905ef5458..1b34e4624701 100644
--- a/tests/unit/script/size_of.rs
+++ b/tests/unit/script/size_of.rs
@@ -38,11 +38,11 @@ macro_rules! sizeof_checker (
 );
 
 // Update the sizes here
-sizeof_checker!(size_event_target, EventTarget, 48);
-sizeof_checker!(size_node, Node, 184);
-sizeof_checker!(size_element, Element, 296);
-sizeof_checker!(size_htmlelement, HTMLElement, 312);
-sizeof_checker!(size_div, HTMLDivElement, 312);
-sizeof_checker!(size_span, HTMLSpanElement, 312);
-sizeof_checker!(size_text, Text, 216);
-sizeof_checker!(size_characterdata, CharacterData, 216);
+sizeof_checker!(size_event_target, EventTarget, 40);
+sizeof_checker!(size_node, Node, 168);
+sizeof_checker!(size_element, Element, 280);
+sizeof_checker!(size_htmlelement, HTMLElement, 296);
+sizeof_checker!(size_div, HTMLDivElement, 296);
+sizeof_checker!(size_span, HTMLSpanElement, 296);
+sizeof_checker!(size_text, Text, 200);
+sizeof_checker!(size_characterdata, CharacterData, 200);