diff --git a/components/script/dom/create.rs b/components/script/dom/create.rs
index fa05f4e00f14..f1895e8844e6 100644
--- a/components/script/dom/create.rs
+++ b/components/script/dom/create.rs
@@ -122,7 +122,7 @@ fn create_html_element(name: QualName,
     assert!(name.ns == ns!(html));
 
     // Step 4
-    let definition = document.lookup_custom_element_definition(name.local.clone(), is);
+    let definition = document.lookup_custom_element_definition(&name.ns, &name.local, is.as_ref());
 
     if let Some(definition) = definition {
         if definition.is_autonomous() {
diff --git a/components/script/dom/customelementregistry.rs b/components/script/dom/customelementregistry.rs
index e3fbe6ef2f52..45482d29c829 100644
--- a/components/script/dom/customelementregistry.rs
+++ b/components/script/dom/customelementregistry.rs
@@ -79,13 +79,13 @@ impl CustomElementRegistry {
 
     /// https://html.spec.whatwg.org/multipage/#look-up-a-custom-element-definition
     pub fn lookup_definition(&self,
-                             local_name: LocalName,
-                             is: Option<LocalName>)
+                             local_name: &LocalName,
+                             is: Option<&LocalName>)
                              -> Option<Rc<CustomElementDefinition>> {
         self.definitions.borrow().values().find(|definition| {
             // Step 4-5
-            definition.local_name == local_name &&
-                (definition.name == local_name || Some(&definition.name) == is.as_ref())
+            definition.local_name == *local_name &&
+                (definition.name == *local_name || Some(&definition.name) == is)
         }).cloned()
     }
 
@@ -522,7 +522,10 @@ fn upgrade_element(definition: Rc<CustomElementDefinition>, element: &Element) {
 pub fn try_upgrade_element(element: &Element) {
     // Step 1
     let document = document_from_node(element);
-    if let Some(definition) = document.lookup_custom_element_definition(element.local_name().clone(), element.get_is()) {
+    let namespace = element.namespace();
+    let local_name = element.local_name();
+    let is = element.get_is();
+    if let Some(definition) = document.lookup_custom_element_definition(namespace, local_name, is.as_ref()) {
         // Step 2
         ScriptThread::enqueue_upgrade_reaction(element, definition);
     }
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index c63295a06744..4f594f96357b 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -95,7 +95,7 @@ use dom_struct::dom_struct;
 use encoding::EncodingRef;
 use encoding::all::UTF_8;
 use euclid::{Point2D, Vector2D};
-use html5ever::{LocalName, QualName};
+use html5ever::{LocalName, Namespace, QualName};
 use hyper::header::{Header, SetCookie};
 use hyper_serde::Serde;
 use ipc_channel::ipc::{self, IpcSender};
@@ -2001,13 +2001,19 @@ impl Document {
 
     /// https://html.spec.whatwg.org/multipage/#look-up-a-custom-element-definition
     pub fn lookup_custom_element_definition(&self,
-                                            local_name: LocalName,
-                                            is: Option<LocalName>)
+                                            namespace: &Namespace,
+                                            local_name: &LocalName,
+                                            is: Option<&LocalName>)
                                             -> Option<Rc<CustomElementDefinition>> {
         if !PREFS.get("dom.customelements.enabled").as_boolean().unwrap_or(false) {
             return None;
         }
 
+        // Step 1
+        if *namespace != ns!(html) {
+            return None;
+        }
+
         // Step 2
         if !self.has_browsing_context {
             return None;