diff --git a/components/script/dom/create.rs b/components/script/dom/create.rs
index b96406b60ed..e22e28a886e 100644
--- a/components/script/dom/create.rs
+++ b/components/script/dom/create.rs
@@ -2,10 +2,12 @@
  * 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/. */
 
+use dom::bindings::error::{report_pending_exception, throw_dom_exception};
 use dom::bindings::js::Root;
+use dom::bindings::reflector::DomObject;
 use dom::document::Document;
-use dom::element::Element;
-use dom::element::ElementCreator;
+use dom::element::{CustomElementCreationMode, Element, ElementCreator};
+use dom::globalscope::GlobalScope;
 use dom::htmlanchorelement::HTMLAnchorElement;
 use dom::htmlappletelement::HTMLAppletElement;
 use dom::htmlareaelement::HTMLAreaElement;
@@ -76,7 +78,8 @@ use dom::htmlulistelement::HTMLUListElement;
 use dom::htmlunknownelement::HTMLUnknownElement;
 use dom::htmlvideoelement::HTMLVideoElement;
 use dom::svgsvgelement::SVGSVGElement;
-use html5ever::{QualName, Prefix};
+use html5ever::{LocalName, Prefix, QualName};
+use js::jsapi::JSAutoCompartment;
 use servo_config::prefs::PREFS;
 
 fn create_svg_element(name: QualName,
@@ -106,11 +109,55 @@ fn create_svg_element(name: QualName,
     }
 }
 
+// https://dom.spec.whatwg.org/#concept-create-element
+#[allow(unsafe_code)]
 fn create_html_element(name: QualName,
                        prefix: Option<Prefix>,
+                       is: Option<LocalName>,
                        document: &Document,
-                       creator: ElementCreator)
+                       creator: ElementCreator,
+                       mode: CustomElementCreationMode)
                        -> Root<Element> {
+    assert!(name.ns == ns!(html));
+
+    // Step 4
+    let definition = document.lookup_custom_element_definition(name.local.clone(), is);
+
+    if let Some(definition) = definition {
+        if definition.is_autonomous() {
+            match mode {
+                // TODO: Handle asynchronous CE creation. Relies on CE upgrades.
+                CustomElementCreationMode::Asynchronous => {},
+                CustomElementCreationMode::Synchronous => {
+                    let local_name = name.local.clone();
+                    return match definition.create_element(document, prefix.clone()) {
+                        Ok(element) => element,
+                        Err(error) => {
+                            // Step 6. Recovering from exception.
+                            let global = GlobalScope::current().unwrap_or_else(|| document.global());
+                            let cx = global.get_cx();
+
+                            // Step 6.1.1
+                            unsafe {
+                                let _ac = JSAutoCompartment::new(cx, global.reflector().get_jsobject().get());
+                                throw_dom_exception(cx, &global, error);
+                                report_pending_exception(cx, true);
+                            }
+
+                            // Step 6.1.2
+                            Root::upcast(HTMLUnknownElement::new(local_name, prefix, document))
+                        },
+                    };
+                },
+            }
+        } else {
+            let element = create_native_html_element(name, prefix, document, creator);
+            element.set_is(definition.name.clone());
+            // TODO: Enqueue custom element upgrade
+            return element;
+        }
+    }
+
     create_native_html_element(name, prefix, document, creator)
 }
 
@@ -281,12 +328,14 @@ pub fn create_native_html_element(name: QualName,
 }
 
 pub fn create_element(name: QualName,
+                      is: Option<LocalName>,
                       document: &Document,
-                      creator: ElementCreator)
+                      creator: ElementCreator,
+                      mode: CustomElementCreationMode)
                       -> Root<Element> {
     let prefix = name.prefix.clone();
     match name.ns {
-        ns!(html)   => create_html_element(name, prefix, document, creator),
+        ns!(html)   => create_html_element(name, prefix, is, document, creator, mode),
         ns!(svg)    => create_svg_element(name, prefix, document),
         _           => Element::new(name.local, name.ns, prefix, document)
     }
diff --git a/components/script/dom/customelementregistry.rs b/components/script/dom/customelementregistry.rs
index ee584a8bae1..94f580f7be9 100644
--- a/components/script/dom/customelementregistry.rs
+++ b/components/script/dom/customelementregistry.rs
@@ -7,23 +7,31 @@ use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::CustomElementRegistryBinding;
 use dom::bindings::codegen::Bindings::CustomElementRegistryBinding::CustomElementRegistryMethods;
 use dom::bindings::codegen::Bindings::CustomElementRegistryBinding::ElementDefinitionOptions;
+use dom::bindings::codegen::Bindings::ElementBinding::ElementMethods;
 use dom::bindings::codegen::Bindings::FunctionBinding::Function;
-use dom::bindings::error::{Error, ErrorResult};
+use dom::bindings::conversions::{ConversionResult, FromJSValConvertible};
+use dom::bindings::error::{Error, ErrorResult, Fallible};
 use dom::bindings::inheritance::Castable;
 use dom::bindings::js::{JS, Root};
 use dom::bindings::reflector::{DomObject, Reflector, reflect_dom_object};
 use dom::bindings::str::DOMString;
+use dom::document::Document;
 use dom::domexception::{DOMErrorName, DOMException};
+use dom::element::Element;
 use dom::globalscope::GlobalScope;
+use dom::htmlelement::HTMLElement;
+use dom::node::Node;
 use dom::promise::Promise;
 use dom::window::Window;
 use dom_struct::dom_struct;
-use html5ever::LocalName;
+use html5ever::{LocalName, Prefix};
 use js::conversions::ToJSValConvertible;
-use js::jsapi::{IsConstructor, HandleObject, JS_GetProperty, JSAutoCompartment, JSContext};
-use js::jsval::{JSVal, UndefinedValue};
+use js::jsapi::{Construct1, IsConstructor, HandleValueArray, HandleObject};
+use js::jsapi::{JS_GetProperty, JSAutoCompartment, JSContext};
+use js::jsval::{JSVal, ObjectValue, UndefinedValue};
 use std::cell::Cell;
 use std::collections::HashMap;
+use std::ptr;
 use std::rc::Rc;
 
 /// https://html.spec.whatwg.org/multipage/#customelementregistry
@@ -34,12 +42,12 @@ pub struct CustomElementRegistry {
     window: JS<Window>,
 
     #[ignore_heap_size_of = "Rc"]
-    when_defined: DOMRefCell<HashMap<DOMString, Rc<Promise>>>,
+    when_defined: DOMRefCell<HashMap<LocalName, Rc<Promise>>>,
 
     element_definition_is_running: Cell<bool>,
 
     #[ignore_heap_size_of = "Rc"]
-    definitions: DOMRefCell<HashMap<DOMString, Rc<CustomElementDefinition>>>,
+    definitions: DOMRefCell<HashMap<LocalName, Rc<CustomElementDefinition>>>,
 }
 
 impl CustomElementRegistry {
@@ -65,6 +73,18 @@ impl CustomElementRegistry {
         self.when_defined.borrow_mut().clear()
     }
 
+    /// https://html.spec.whatwg.org/multipage/#look-up-a-custom-element-definition
+    pub fn lookup_definition(&self,
+                             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())
+        }).cloned()
+    }
+
     pub fn lookup_definition_by_constructor(&self, constructor: HandleObject) -> Option<Rc<CustomElementDefinition>> {
         self.definitions.borrow().values().find(|definition| {
             definition.constructor.callback() == constructor.get()
@@ -101,6 +121,7 @@ impl CustomElementRegistryMethods for CustomElementRegistry {
     fn Define(&self, name: DOMString, constructor_: Rc<Function>, options: &ElementDefinitionOptions) -> ErrorResult {
         let global_scope = self.window.upcast::<GlobalScope>();
         rooted!(in(global_scope.get_cx()) let constructor = constructor_.callback());
+        let name = LocalName::from(&*name);
 
         // Step 1
         if unsafe { !IsConstructor(constructor.get()) } {
@@ -137,10 +158,10 @@ impl CustomElementRegistryMethods for CustomElementRegistry {
                 return Err(Error::NotSupported)
             }
 
-            extended_name
+            LocalName::from(&**extended_name)
         } else {
             // Step 7.3
-            &name
+            name.clone()
         };
 
         // Step 8
@@ -165,8 +186,8 @@ impl CustomElementRegistryMethods for CustomElementRegistry {
         result?;
 
         // Step 11
-        let definition = CustomElementDefinition::new(LocalName::from(&*name),
-                                                      LocalName::from(&**local_name),
+        let definition = CustomElementDefinition::new(name.clone(),
+                                                      local_name,
                                                       constructor_);
 
         // Step 12
@@ -188,7 +209,7 @@ impl CustomElementRegistryMethods for CustomElementRegistry {
     /// https://html.spec.whatwg.org/multipage/#dom-customelementregistry-get
     #[allow(unsafe_code)]
     unsafe fn Get(&self, cx: *mut JSContext, name: DOMString) -> JSVal {
-        match self.definitions.borrow().get(&name) {
+        match self.definitions.borrow().get(&LocalName::from(&*name)) {
             Some(definition) => {
                 rooted!(in(cx) let mut constructor = UndefinedValue());
                 definition.constructor.to_jsval(cx, constructor.handle_mut());
@@ -202,6 +223,7 @@ impl CustomElementRegistryMethods for CustomElementRegistry {
     #[allow(unrooted_must_root)]
     fn WhenDefined(&self, name: DOMString) -> Rc<Promise> {
         let global_scope = self.window.upcast::<GlobalScope>();
+        let name = LocalName::from(&*name);
 
         // Step 1
         if !is_valid_custom_element_name(&name) {
@@ -256,6 +278,56 @@ impl CustomElementDefinition {
     pub fn is_autonomous(&self) -> bool {
         self.name == self.local_name
     }
+
+    /// https://dom.spec.whatwg.org/#concept-create-element Step 6.1
+    #[allow(unsafe_code)]
+    pub fn create_element(&self, document: &Document, prefix: Option<Prefix>) -> Fallible<Root<Element>> {
+        let window = document.window();
+        let cx = window.get_cx();
+        // Step 2
+        rooted!(in(cx) let constructor = ObjectValue(self.constructor.callback()));
+        rooted!(in(cx) let mut element = ptr::null_mut());
+        {
+            // Go into the constructor's compartment
+            let _ac = JSAutoCompartment::new(cx, self.constructor.callback());
+            let args = HandleValueArray::new();
+            if unsafe { !Construct1(cx, constructor.handle(), &args, element.handle_mut()) } {
+                return Err(Error::JSFailed);
+            }
+        }
+
+        rooted!(in(cx) let element_val = ObjectValue(element.get()));
+        let element: Root<Element> = match unsafe { Root::from_jsval(cx, element_val.handle(), ()) } {
+            Ok(ConversionResult::Success(element)) => element,
+            Ok(ConversionResult::Failure(..)) =>
+                return Err(Error::Type("Constructor did not return a DOM node".to_owned())),
+            _ => return Err(Error::JSFailed),
+        };
+
+        // Step 3
+        if !element.is::<HTMLElement>() {
+            return Err(Error::Type("Constructor did not return a DOM node".to_owned()));
+        }
+
+        // Steps 4-9
+        if element.HasAttributes() ||
+            element.upcast::<Node>().children_count() > 0 ||
+            element.upcast::<Node>().has_parent() ||
+            &*element.upcast::<Node>().owner_doc() != document ||
+            *element.namespace() != ns!(html) ||
+            *element.local_name() != self.local_name
+        {
+            return Err(Error::NotSupported);
+        }
+
+        // Step 10
+        element.set_prefix(prefix);
+
+        // Step 11
+        // Element's `is` is None by default
+
+        Ok(element)
+    }
 }
 
 /// https://html.spec.whatwg.org/multipage/#valid-custom-element-name
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index c97d352872c..60d4f52ef6b 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -13,7 +13,7 @@ use dom::bindings::callback::ExceptionHandling;
 use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::DOMRectBinding::DOMRectMethods;
 use dom::bindings::codegen::Bindings::DocumentBinding;
-use dom::bindings::codegen::Bindings::DocumentBinding::{DocumentMethods, DocumentReadyState};
+use dom::bindings::codegen::Bindings::DocumentBinding::{DocumentMethods, DocumentReadyState, ElementCreationOptions};
 use dom::bindings::codegen::Bindings::ElementBinding::ElementMethods;
 use dom::bindings::codegen::Bindings::EventHandlerBinding::EventHandlerNonNull;
 use dom::bindings::codegen::Bindings::EventHandlerBinding::OnErrorEventHandlerNonNull;
@@ -36,11 +36,13 @@ use dom::bindings::xmlname::{namespace_from_domstring, validate_and_extract, xml
 use dom::bindings::xmlname::XMLName::InvalidXMLName;
 use dom::closeevent::CloseEvent;
 use dom::comment::Comment;
+use dom::customelementregistry::CustomElementDefinition;
 use dom::customevent::CustomEvent;
 use dom::documentfragment::DocumentFragment;
 use dom::documenttype::DocumentType;
 use dom::domimplementation::DOMImplementation;
 use dom::element::{Element, ElementCreator, ElementPerformFullscreenEnter, ElementPerformFullscreenExit};
+use dom::element::CustomElementCreationMode;
 use dom::errorevent::ErrorEvent;
 use dom::event::{Event, EventBubbles, EventCancelable, EventDefault, EventStatus};
 use dom::eventtarget::EventTarget;
@@ -1996,6 +1998,26 @@ impl Document {
 
         self.window.layout().nodes_from_point_response()
     }
+
+    /// 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>)
+                                            -> Option<Rc<CustomElementDefinition>> {
+        if !PREFS.get("dom.customelements.enabled").as_boolean().unwrap_or(false) {
+            return None;
+        }
+
+        // Step 2
+        if !self.has_browsing_context {
+            return None;
+        }
+
+        // Step 3
+        let registry = self.window.CustomElements();
+
+        registry.lookup_definition(local_name, is)
+    }
 }
 
 #[derive(PartialEq, HeapSizeOf)]
@@ -2813,7 +2835,10 @@ impl DocumentMethods for Document {
     }
 
     // https://dom.spec.whatwg.org/#dom-document-createelement
-    fn CreateElement(&self, mut local_name: DOMString) -> Fallible<Root<Element>> {
+    fn CreateElement(&self,
+                     mut local_name: DOMString,
+                     options: &ElementCreationOptions)
+                     -> Fallible<Root<Element>> {
         if xml_name_type(&local_name) == InvalidXMLName {
             debug!("Not a valid element name");
             return Err(Error::InvalidCharacter);
@@ -2829,18 +2854,21 @@ impl DocumentMethods for Document {
         };
 
         let name = QualName::new(None, ns, LocalName::from(local_name));
-        Ok(Element::create(name, self, ElementCreator::ScriptCreated))
+        let is = options.is.as_ref().map(|is| LocalName::from(&**is));
+        Ok(Element::create(name, is, self, ElementCreator::ScriptCreated, CustomElementCreationMode::Synchronous))
     }
 
     // https://dom.spec.whatwg.org/#dom-document-createelementns
     fn CreateElementNS(&self,
                        namespace: Option<DOMString>,
-                       qualified_name: DOMString)
+                       qualified_name: DOMString,
+                       options: &ElementCreationOptions)
                        -> Fallible<Root<Element>> {
         let (namespace, prefix, local_name) = validate_and_extract(namespace,
                                                                         &qualified_name)?;
         let name = QualName::new(prefix, namespace, local_name);
-        Ok(Element::create(name, self, ElementCreator::ScriptCreated))
+        let is = options.is.as_ref().map(|is| LocalName::from(&**is));
+        Ok(Element::create(name, is, self, ElementCreator::ScriptCreated, CustomElementCreationMode::Synchronous))
     }
 
     // https://dom.spec.whatwg.org/#dom-document-createattribute
@@ -3094,7 +3122,11 @@ impl DocumentMethods for Document {
                 Some(elem) => Root::upcast::<Node>(elem),
                 None => {
                     let name = QualName::new(None, ns!(svg), local_name!("title"));
-                    let elem = Element::create(name, self, ElementCreator::ScriptCreated);
+                    let elem = Element::create(name,
+                                               None,
+                                               self,
+                                               ElementCreator::ScriptCreated,
+                                               CustomElementCreationMode::Synchronous);
                     let parent = root.upcast::<Node>();
                     let child = elem.upcast::<Node>();
                     parent.InsertBefore(child, parent.GetFirstChild().r())
@@ -3112,8 +3144,10 @@ impl DocumentMethods for Document {
                         Some(head) => {
                             let name = QualName::new(None, ns!(html), local_name!("title"));
                             let elem = Element::create(name,
+                                                       None,
                                                        self,
-                                                       ElementCreator::ScriptCreated);
+                                                       ElementCreator::ScriptCreated,
+                                                       CustomElementCreationMode::Synchronous);
                             head.upcast::<Node>()
                                 .AppendChild(elem.upcast())
                                 .unwrap()
diff --git a/components/script/dom/domimplementation.rs b/components/script/dom/domimplementation.rs
index 8c1bee4f334..5b56242e14b 100644
--- a/components/script/dom/domimplementation.rs
+++ b/components/script/dom/domimplementation.rs
@@ -5,7 +5,7 @@
 use document_loader::DocumentLoader;
 use dom::bindings::codegen::Bindings::DOMImplementationBinding;
 use dom::bindings::codegen::Bindings::DOMImplementationBinding::DOMImplementationMethods;
-use dom::bindings::codegen::Bindings::DocumentBinding::DocumentMethods;
+use dom::bindings::codegen::Bindings::DocumentBinding::{DocumentMethods, ElementCreationOptions};
 use dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
 use dom::bindings::error::Fallible;
 use dom::bindings::inheritance::Castable;
@@ -92,7 +92,8 @@ impl DOMImplementationMethods for DOMImplementation {
         let maybe_elem = if qname.is_empty() {
             None
         } else {
-            match doc.upcast::<Document>().CreateElementNS(maybe_namespace, qname) {
+            let options = ElementCreationOptions { is: None };
+            match doc.upcast::<Document>().CreateElementNS(maybe_namespace, qname, &options) {
                 Err(error) => return Err(error),
                 Ok(elem) => Some(elem),
             }
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 2c7372ebddf..a78520426e0 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -126,9 +126,10 @@ pub struct Element {
     local_name: LocalName,
     tag_name: TagName,
     namespace: Namespace,
-    prefix: Option<Prefix>,
+    prefix: DOMRefCell<Option<Prefix>>,
     attrs: DOMRefCell<Vec<JS<Attr>>>,
     id_attribute: DOMRefCell<Option<Atom>>,
+    is: DOMRefCell<Option<LocalName>>,
     #[ignore_heap_size_of = "Arc"]
     style_attribute: DOMRefCell<Option<Arc<Locked<PropertyDeclarationBlock>>>>,
     attr_list: MutNullableJS<NamedNodeMap>,
@@ -164,6 +165,11 @@ pub enum ElementCreator {
     ScriptCreated,
 }
 
+pub enum CustomElementCreationMode {
+    Synchronous,
+    Asynchronous,
+}
+
 impl ElementCreator {
     pub fn is_parser_created(&self) -> bool {
         match *self {
@@ -205,9 +211,12 @@ impl<'a> TryFrom<&'a str> for AdjacentPosition {
 //
 impl Element {
     pub fn create(name: QualName,
-                  document: &Document, creator: ElementCreator)
+                  is: Option<LocalName>,
+                  document: &Document,
+                  creator: ElementCreator,
+                  mode: CustomElementCreationMode)
                   -> Root<Element> {
-        create_element(name, document, creator)
+        create_element(name, is, document, creator, mode)
     }
 
     pub fn new_inherited(local_name: LocalName,
@@ -226,9 +235,10 @@ impl Element {
             local_name: local_name,
             tag_name: TagName::new(),
             namespace: namespace,
-            prefix: prefix,
+            prefix: DOMRefCell::new(prefix),
             attrs: DOMRefCell::new(vec![]),
             id_attribute: DOMRefCell::new(None),
+            is: DOMRefCell::new(None),
             style_attribute: DOMRefCell::new(None),
             attr_list: Default::default(),
             class_list: Default::default(),
@@ -260,6 +270,14 @@ impl Element {
         }
     }
 
+    pub fn set_is(&self, is: LocalName) {
+        *self.is.borrow_mut() = Some(is);
+    }
+
+    pub fn get_is(&self) -> Option<LocalName> {
+        self.is.borrow().clone()
+    }
+
     // https://drafts.csswg.org/cssom-view/#css-layout-box
     // Elements that have a computed value of the display property
     // that is table-column or table-column-group
@@ -820,8 +838,12 @@ impl Element {
         &self.namespace
     }
 
-    pub fn prefix(&self) -> Option<&Prefix> {
-        self.prefix.as_ref()
+    pub fn prefix(&self) -> Ref<Option<Prefix>> {
+        self.prefix.borrow()
+    }
+
+    pub fn set_prefix(&self, prefix: Option<Prefix>) {
+        *self.prefix.borrow_mut() = prefix;
     }
 
     pub fn attrs(&self) -> Ref<[JS<Attr>]> {
@@ -840,7 +862,9 @@ impl Element {
         // Steps 3-4.
         for element in inclusive_ancestor_elements {
             // Step 1.
-            if element.namespace() != &ns!() && element.prefix().map(|p| &**p) == prefix.as_ref().map(|p| &**p) {
+            if element.namespace() != &ns!() &&
+                element.prefix().as_ref().map(|p| &**p) == prefix.as_ref().map(|p| &**p)
+            {
                 return element.namespace().clone();
             }
 
@@ -1418,13 +1442,13 @@ impl ElementMethods for Element {
 
     // https://dom.spec.whatwg.org/#dom-element-prefix
     fn GetPrefix(&self) -> Option<DOMString> {
-        self.prefix.as_ref().map(|p| DOMString::from(&**p))
+        self.prefix.borrow().as_ref().map(|p| DOMString::from(&**p))
     }
 
     // https://dom.spec.whatwg.org/#dom-element-tagname
     fn TagName(&self) -> DOMString {
         let name = self.tag_name.or_init(|| {
-            let qualified_name = match self.prefix {
+            let qualified_name = match *self.prefix.borrow() {
                 Some(ref prefix) => {
                     Cow::Owned(format!("{}:{}", &**prefix, &*self.local_name))
                 },
@@ -1970,8 +1994,10 @@ impl ElementMethods for Element {
             // Step 4.
             NodeTypeId::DocumentFragment => {
                 let body_elem = Element::create(QualName::new(None, ns!(html), local_name!("body")),
+                                                None,
                                                 &context_document,
-                                                ElementCreator::ScriptCreated);
+                                                ElementCreator::ScriptCreated,
+                                                CustomElementCreationMode::Synchronous);
                 Root::upcast(body_elem)
             },
             _ => context_node.GetParentElement().unwrap()
diff --git a/components/script/dom/htmlcollection.rs b/components/script/dom/htmlcollection.rs
index 6cc3dbb8dc6..b9c402b3003 100644
--- a/components/script/dom/htmlcollection.rs
+++ b/components/script/dom/htmlcollection.rs
@@ -152,7 +152,7 @@ impl HTMLCollection {
     }
 
     fn match_element(elem: &Element, qualified_name: &LocalName) -> bool {
-        match elem.prefix() {
+        match elem.prefix().as_ref() {
             None => elem.local_name() == qualified_name,
             Some(prefix) => qualified_name.starts_with(&**prefix) &&
                 qualified_name.find(":") == Some(prefix.len()) &&
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index cace0497433..2e0152195dd 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -33,7 +33,7 @@ use dom::cssstylesheet::CSSStyleSheet;
 use dom::document::{Document, DocumentSource, HasBrowsingContext, IsHTMLDocument};
 use dom::documentfragment::DocumentFragment;
 use dom::documenttype::DocumentType;
-use dom::element::{Element, ElementCreator};
+use dom::element::{CustomElementCreationMode, Element, ElementCreator};
 use dom::eventtarget::EventTarget;
 use dom::globalscope::GlobalScope;
 use dom::htmlbodyelement::HTMLBodyElement;
@@ -428,6 +428,11 @@ impl Node {
         self.preceding_siblings().count() as u32
     }
 
+    /// Returns true if this node has a parent.
+    pub fn has_parent(&self) -> bool {
+        self.parent_node.get().is_some()
+    }
+
     pub fn children_count(&self) -> u32 {
         self.children_count.get()
     }
@@ -1818,12 +1823,15 @@ impl Node {
             NodeTypeId::Element(..) => {
                 let element = node.downcast::<Element>().unwrap();
                 let name = QualName {
-                    prefix: element.prefix().map(|p| Prefix::from(&**p)),
+                    prefix: element.prefix().as_ref().map(|p| Prefix::from(&**p)),
                     ns: element.namespace().clone(),
                     local: element.local_name().clone()
                 };
                 let element = Element::create(name,
-                    &document, ElementCreator::ScriptCreated);
+                                              element.get_is(),
+                                              &document,
+                                              ElementCreator::ScriptCreated,
+                                              CustomElementCreationMode::Asynchronous);
                 Root::upcast::<Node>(element)
             },
         };
@@ -2284,7 +2292,7 @@ impl NodeMethods for Node {
             let element = node.downcast::<Element>().unwrap();
             let other_element = other.downcast::<Element>().unwrap();
             (*element.namespace() == *other_element.namespace()) &&
-            (element.prefix() == other_element.prefix()) &&
+            (*element.prefix() == *other_element.prefix()) &&
             (*element.local_name() == *other_element.local_name()) &&
             (element.attrs().len() == other_element.attrs().len())
         }
diff --git a/components/script/dom/servoparser/async_html.rs b/components/script/dom/servoparser/async_html.rs
index e1632ffaf3c..a7ac04aa6de 100644
--- a/components/script/dom/servoparser/async_html.rs
+++ b/components/script/dom/servoparser/async_html.rs
@@ -13,14 +13,14 @@ use dom::bindings::trace::JSTraceable;
 use dom::comment::Comment;
 use dom::document::Document;
 use dom::documenttype::DocumentType;
-use dom::element::{Element, ElementCreator};
+use dom::element::{CustomElementCreationMode, Element, ElementCreator};
 use dom::htmlformelement::{FormControlElementHelpers, HTMLFormElement};
 use dom::htmlscriptelement::HTMLScriptElement;
 use dom::htmltemplateelement::HTMLTemplateElement;
 use dom::node::Node;
 use dom::processinginstruction::ProcessingInstruction;
 use dom::virtualmethods::vtable_for;
-use html5ever::{Attribute, QualName, ExpandedName};
+use html5ever::{Attribute, LocalName, QualName, ExpandedName};
 use html5ever::buffer_queue::BufferQueue;
 use html5ever::tendril::StrTendril;
 use html5ever::tokenizer::{Tokenizer as HtmlTokenizer, TokenizerOpts, TokenizerResult};
@@ -245,8 +245,15 @@ impl Sink {
                 self.insert_node(contents, JS::from_ref(template.Content().upcast()));
             }
             ParseOperation::CreateElement(id, name, attrs) => {
-                let elem = Element::create(name, &*self.document,
-                                           ElementCreator::ParserCreated(self.current_line));
+                let is = attrs.iter()
+                              .find(|attr| attr.name.local.eq_str_ignore_ascii_case("is"))
+                              .map(|attr| LocalName::from(&*attr.value));
+
+                let elem = Element::create(name,
+                                           is,
+                                           &*self.document,
+                                           ElementCreator::ParserCreated(self.current_line),
+                                           CustomElementCreationMode::Synchronous);
                 for attr in attrs {
                     elem.set_attribute_from_parser(attr.name, DOMString::from(String::from(attr.value)), None);
                 }
diff --git a/components/script/dom/servoparser/mod.rs b/components/script/dom/servoparser/mod.rs
index 6d6edb6d7a8..6fc611b0108 100644
--- a/components/script/dom/servoparser/mod.rs
+++ b/components/script/dom/servoparser/mod.rs
@@ -18,7 +18,7 @@ use dom::characterdata::CharacterData;
 use dom::comment::Comment;
 use dom::document::{Document, DocumentSource, HasBrowsingContext, IsHTMLDocument};
 use dom::documenttype::DocumentType;
-use dom::element::{Element, ElementCreator};
+use dom::element::{Element, ElementCreator, CustomElementCreationMode};
 use dom::globalscope::GlobalScope;
 use dom::htmlformelement::{FormControlElementHelpers, HTMLFormElement};
 use dom::htmlimageelement::HTMLImageElement;
@@ -29,7 +29,7 @@ use dom::processinginstruction::ProcessingInstruction;
 use dom::text::Text;
 use dom::virtualmethods::vtable_for;
 use dom_struct::dom_struct;
-use html5ever::{Attribute, QualName, ExpandedName};
+use html5ever::{Attribute, ExpandedName, LocalName, QualName};
 use html5ever::buffer_queue::BufferQueue;
 use html5ever::tendril::{StrTendril, ByteTendril, IncompleteUtf8};
 use html5ever::tree_builder::{NodeOrText, TreeSink, NextParserState, QuirksMode, ElementFlags};
@@ -782,8 +782,15 @@ impl TreeSink for Sink {
 
     fn create_element(&mut self, name: QualName, attrs: Vec<Attribute>, _flags: ElementFlags)
             -> JS<Node> {
-        let elem = Element::create(name, &*self.document,
-                                   ElementCreator::ParserCreated(self.current_line));
+        let is = attrs.iter()
+                      .find(|attr| attr.name.local.eq_str_ignore_ascii_case("is"))
+                      .map(|attr| LocalName::from(&*attr.value));
+
+        let elem = Element::create(name,
+                                   is,
+                                   &*self.document,
+                                   ElementCreator::ParserCreated(self.current_line),
+                                   CustomElementCreationMode::Synchronous);
 
         for attr in attrs {
             elem.set_attribute_from_parser(attr.name, DOMString::from(String::from(attr.value)), None);
diff --git a/components/script/dom/webidls/Document.webidl b/components/script/dom/webidls/Document.webidl
index e3c67851011..457d92394d2 100644
--- a/components/script/dom/webidls/Document.webidl
+++ b/components/script/dom/webidls/Document.webidl
@@ -33,9 +33,9 @@ interface Document : Node {
   HTMLCollection getElementsByClassName(DOMString classNames);
 
   [NewObject, Throws]
-  Element createElement(DOMString localName);
+  Element createElement(DOMString localName, optional ElementCreationOptions options);
   [NewObject, Throws]
-  Element createElementNS(DOMString? namespace, DOMString qualifiedName);
+  Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional ElementCreationOptions options);
   [NewObject]
   DocumentFragment createDocumentFragment();
   [NewObject]
@@ -75,6 +75,10 @@ Document implements ParentNode;
 
 enum DocumentReadyState { "loading", "interactive", "complete" };
 
+dictionary ElementCreationOptions {
+  DOMString is;
+};
+
 // https://html.spec.whatwg.org/multipage/#the-document-object
 // [OverrideBuiltins]
 partial /*sealed*/ interface Document {
diff --git a/tests/unit/script/size_of.rs b/tests/unit/script/size_of.rs
index c7c04b4aac1..c4d5c939663 100644
--- a/tests/unit/script/size_of.rs
+++ b/tests/unit/script/size_of.rs
@@ -31,10 +31,10 @@ macro_rules! sizeof_checker (
 // Update the sizes here
 sizeof_checker!(size_event_target, EventTarget, 40);
 sizeof_checker!(size_node, Node, 184);
-sizeof_checker!(size_element, Element, 344);
-sizeof_checker!(size_htmlelement, HTMLElement, 360);
-sizeof_checker!(size_div, HTMLDivElement, 360);
-sizeof_checker!(size_span, HTMLSpanElement, 360);
+sizeof_checker!(size_element, Element, 376);
+sizeof_checker!(size_htmlelement, HTMLElement, 392);
+sizeof_checker!(size_div, HTMLDivElement, 392);
+sizeof_checker!(size_span, HTMLSpanElement, 392);
 sizeof_checker!(size_text, Text, 216);
 sizeof_checker!(size_characterdata, CharacterData, 216);
 sizeof_checker!(size_servothreadsafelayoutnode, ServoThreadSafeLayoutNode, 16);
diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index bb4ad32115b..58ee872a951 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -554206,7 +554206,7 @@
    "testharness"
   ],
   "custom-elements/Document-createElement.html": [
-   "d9582ab3867b02e98da9b0da8e3398303d1833ab",
+   "074c9f703cc7feb1dfc3d07aedd08460bd591d42",
    "testharness"
   ],
   "custom-elements/HTMLElement-constructor.html": [
diff --git a/tests/wpt/metadata/custom-elements/CustomElementRegistry.html.ini b/tests/wpt/metadata/custom-elements/CustomElementRegistry.html.ini
index e99a0cb2bdb..72117493595 100644
--- a/tests/wpt/metadata/custom-elements/CustomElementRegistry.html.ini
+++ b/tests/wpt/metadata/custom-elements/CustomElementRegistry.html.ini
@@ -1,8 +1,5 @@
 [CustomElementRegistry.html]
   type: testharness
-  [customElements.define must not throw when defining another custom element in a different global object during Get(constructor, "prototype")]
-    expected: FAIL
-
   [customElements.define must get callbacks of the constructor prototype]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/Document-createElement.html.ini b/tests/wpt/metadata/custom-elements/Document-createElement.html.ini
deleted file mode 100644
index 9978606560f..00000000000
--- a/tests/wpt/metadata/custom-elements/Document-createElement.html.ini
+++ /dev/null
@@ -1,38 +0,0 @@
-[Document-createElement.html]
-  type: testharness
-  [document.createElement must create an instance of custom elements]
-    expected: FAIL
-
-  [document.createElement must report a TypeError when the result of Construct is not a DOM node]
-    expected: FAIL
-
-  [document.createElement must report a TypeError when the result of Construct is a TextNode]
-    expected: FAIL
-
-  [document.createElement must report a NotSupportedError when attribute is added by setAttribute during construction]
-    expected: FAIL
-
-  [document.createElement must report a NotSupportedError when attribute is added by attributes.setNamedItem during construction]
-    expected: FAIL
-
-  [document.createElement must not report a NotSupportedError when attribute is added and removed during construction]
-    expected: FAIL
-
-  [document.createElement must report a NotSupportedError when a Text child is added during construction]
-    expected: FAIL
-
-  [document.createElement must report a NotSupportedError when a Comment child is added during construction]
-    expected: FAIL
-
-  [document.createElement must report a NotSupportedError when an element child is added during construction]
-    expected: FAIL
-
-  [document.createElement must not report a NotSupportedError when an element child is added and removed during construction]
-    expected: FAIL
-
-  [document.createElement must report a NotSupportedError when the element gets inserted into another element during construction]
-    expected: FAIL
-
-  [document.createElement must not report a NotSupportedError when the element is inserted and removed from another element during construction]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-in-document-write.html.ini b/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-in-document-write.html.ini
index acf53969d9d..f93d34eb9b8 100644
--- a/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-in-document-write.html.ini
+++ b/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-in-document-write.html.ini
@@ -1,8 +1,5 @@
 [parser-constructs-custom-element-in-document-write.html]
   type: testharness
-  [HTML parser must instantiate custom elements inside document.write]
-    expected: FAIL
-
   [Custom Elements: Changes to the HTML parser]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-synchronously.html.ini b/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-synchronously.html.ini
index 191462ce811..48df95140d3 100644
--- a/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-synchronously.html.ini
+++ b/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-element-synchronously.html.ini
@@ -1,8 +1,5 @@
 [parser-constructs-custom-element-synchronously.html]
   type: testharness
-  [HTML parser must only append nodes that appear before a custom element before instantiating the custom element]
-    expected: FAIL
-
   [Custom Elements: Changes to the HTML parser]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-elements.html.ini b/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-elements.html.ini
deleted file mode 100644
index d6aae45c4a3..00000000000
--- a/tests/wpt/metadata/custom-elements/parser/parser-constructs-custom-elements.html.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[parser-constructs-custom-elements.html]
-  type: testharness
-  [HTML parser must create a defined custom element before executing inline scripts]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/custom-elements/parser/parser-sets-attributes-and-children.html.ini b/tests/wpt/metadata/custom-elements/parser/parser-sets-attributes-and-children.html.ini
index 177da52fce6..51e75dccba0 100644
--- a/tests/wpt/metadata/custom-elements/parser/parser-sets-attributes-and-children.html.ini
+++ b/tests/wpt/metadata/custom-elements/parser/parser-sets-attributes-and-children.html.ini
@@ -1,8 +1,5 @@
 [parser-sets-attributes-and-children.html]
   type: testharness
-  [HTML parser must set the attributes or append children before calling constructor]
-    expected: FAIL
-
   [Custom Elements: Changes to the HTML parser]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/parser/parser-uses-constructed-element.html.ini b/tests/wpt/metadata/custom-elements/parser/parser-uses-constructed-element.html.ini
index 1c940fa0990..f3f6e422571 100644
--- a/tests/wpt/metadata/custom-elements/parser/parser-uses-constructed-element.html.ini
+++ b/tests/wpt/metadata/custom-elements/parser/parser-uses-constructed-element.html.ini
@@ -1,11 +1,5 @@
 [parser-uses-constructed-element.html]
   type: testharness
-  [HTML parser must use the returned value of the custom element constructor instead of the one created before super() call]
-    expected: FAIL
-
-  [HTML parser must use the returned value of the custom element constructor instead using the one created in super() call]
-    expected: FAIL
-
   [Custom Elements: HTML parser must construct a custom element instead of upgrading]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/parser/parser-uses-registry-of-owner-document.html.ini b/tests/wpt/metadata/custom-elements/parser/parser-uses-registry-of-owner-document.html.ini
index bd53f25903e..ef16e6972c8 100644
--- a/tests/wpt/metadata/custom-elements/parser/parser-uses-registry-of-owner-document.html.ini
+++ b/tests/wpt/metadata/custom-elements/parser/parser-uses-registry-of-owner-document.html.ini
@@ -9,3 +9,6 @@
   [Custom Elements: HTML parser must use the owner document's custom element registry]
     expected: FAIL
 
+  [HTML parser must not instantiate custom elements inside template elements]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/custom-elements/reaction-timing.html.ini b/tests/wpt/metadata/custom-elements/reaction-timing.html.ini
index 5b335cb5852..14219a014a4 100644
--- a/tests/wpt/metadata/custom-elements/reaction-timing.html.ini
+++ b/tests/wpt/metadata/custom-elements/reaction-timing.html.ini
@@ -3,9 +3,9 @@
   [setAttribute and removeAttribute must enqueue and invoke attributeChangedCallback]
     expected: FAIL
 
-  [Calling Node.prototype.cloneNode(false) must push a new element queue to the processing stack]
-    expected: FAIL
-
   [Custom Elements: Custom element reactions must be invoked before returning to author scripts]
     expected: FAIL
 
+  [Calling Node.prototype.cloneNode(false) must push a new element queue to the processing stack]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/custom-elements/reactions/Attr.html.ini b/tests/wpt/metadata/custom-elements/reactions/Attr.html.ini
index 5a3ddc4ec85..bd4c3d13679 100644
--- a/tests/wpt/metadata/custom-elements/reactions/Attr.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/Attr.html.ini
@@ -3,6 +3,3 @@
   [value on Attr must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [value on Attr must not enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/custom-elements/reactions/CSSStyleDeclaration.html.ini b/tests/wpt/metadata/custom-elements/reactions/CSSStyleDeclaration.html.ini
index 25a52820a2e..63215ffadfd 100644
--- a/tests/wpt/metadata/custom-elements/reactions/CSSStyleDeclaration.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/CSSStyleDeclaration.html.ini
@@ -3,87 +3,45 @@
   [cssText on CSSStyleDeclaration must enqueue an attributeChanged reaction when it adds the observed style attribute]
     expected: FAIL
 
-  [cssText on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it adds the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [cssText on CSSStyleDeclaration must enqueue an attributeChanged reaction when it mutates the observed style attribute]
     expected: FAIL
 
-  [cssText on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it mutates the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [setProperty on CSSStyleDeclaration must enqueue an attributeChanged reaction when it adds the observed style attribute]
     expected: FAIL
 
-  [setProperty on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it adds the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [setProperty on CSSStyleDeclaration must enqueue an attributeChanged reaction when it mutates the observed style attribute]
     expected: FAIL
 
-  [setProperty on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it mutates the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [setProperty on CSSStyleDeclaration must enqueue an attributeChanged reaction when it makes a property important and the style attribute is observed]
     expected: FAIL
 
-  [setProperty on CSSStyleDeclaration must enqueue an attributeChanged reaction when it makes a property important but the style attribute is not observed]
-    expected: FAIL
-
   [setPropertyValue on CSSStyleDeclaration must enqueue an attributeChanged reaction when it adds the observed style attribute]
     expected: FAIL
 
-  [setPropertyValue on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it adds the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [setPropertyValue on CSSStyleDeclaration must enqueue an attributeChanged reaction when it mutates the observed style attribute]
     expected: FAIL
 
-  [setPropertyValue on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it mutates the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [setPropertyPriority on CSSStyleDeclaration must enqueue an attributeChanged reaction when it makes a property important and the style attribute is observed]
     expected: FAIL
 
-  [setPropertyPriority on CSSStyleDeclaration must enqueue an attributeChanged reaction when it makes a property important but the style attribute is not observed]
-    expected: FAIL
-
   [removeProperty on CSSStyleDeclaration must enqueue an attributeChanged reaction when it removes a property from the observed style attribute]
     expected: FAIL
 
-  [removeProperty on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it removes a property from the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [cssFloat on CSSStyleDeclaration must enqueue an attributeChanged reaction when it adds the observed style attribute]
     expected: FAIL
 
-  [cssFloat on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it adds the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [A camel case attribute (borderWidth) on CSSStyleDeclaration must enqueue an attributeChanged reaction when it adds the observed style attribute]
     expected: FAIL
 
-  [A camel case attribute (borderWidth) on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it adds the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [A camel case attribute (borderWidth) on CSSStyleDeclaration must enqueue an attributeChanged reaction when it mutates the observed style attribute]
     expected: FAIL
 
-  [A camel case attribute (borderWidth) on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it mutates the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [A dashed property (border-width) on CSSStyleDeclaration must enqueue an attributeChanged reaction when it adds the observed style attribute]
     expected: FAIL
 
-  [A dashed property (border-width) on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it adds the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [A dashed property (border-width) on CSSStyleDeclaration must enqueue an attributeChanged reaction when it mutates the observed style attribute]
     expected: FAIL
 
-  [A dashed property (border-width) on CSSStyleDeclaration must not enqueue an attributeChanged reaction when it mutates the style attribute but the style attribute is not observed]
-    expected: FAIL
-
   [A webkit prefixed camel case attribute (webkitFilter) on CSSStyleDeclaration must enqueue an attributeChanged reaction when it adds the observed style attribute]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/reactions/DOMStringMap.html.ini b/tests/wpt/metadata/custom-elements/reactions/DOMStringMap.html.ini
index 9cd64315e1b..f92cc7c1ae7 100644
--- a/tests/wpt/metadata/custom-elements/reactions/DOMStringMap.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/DOMStringMap.html.ini
@@ -3,24 +3,12 @@
   [setter on DOMStringMap must enqueue an attributeChanged reaction when adding an observed data attribute]
     expected: FAIL
 
-  [setter on DOMStringMap must not enqueue an attributeChanged reaction when adding an unobserved data attribute]
-    expected: FAIL
-
   [setter on DOMStringMap must enqueue an attributeChanged reaction when mutating the value of an observed data attribute]
     expected: FAIL
 
   [setter on DOMStringMap must enqueue an attributeChanged reaction when mutating the value of an observed data attribute to the same value]
     expected: FAIL
 
-  [setter on DOMStringMap must not enqueue an attributeChanged reaction when mutating the value of an unobserved data attribute]
-    expected: FAIL
-
   [deleter on DOMStringMap must enqueue an attributeChanged reaction when removing an observed data attribute]
     expected: FAIL
 
-  [deleter on DOMStringMap must not enqueue an attributeChanged reaction when removing an unobserved data attribute]
-    expected: FAIL
-
-  [deleter on DOMStringMap must not enqueue an attributeChanged reaction when it does not remove a data attribute]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/custom-elements/reactions/DOMTokenList.html.ini b/tests/wpt/metadata/custom-elements/reactions/DOMTokenList.html.ini
index 5c7a1734d34..471f7e8fccf 100644
--- a/tests/wpt/metadata/custom-elements/reactions/DOMTokenList.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/DOMTokenList.html.ini
@@ -3,15 +3,9 @@
   [add on DOMTokenList must enqueue an attributeChanged reaction when adding an attribute]
     expected: FAIL
 
-  [add on DOMTokenList must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [add on DOMTokenList must enqueue an attributeChanged reaction when adding a value to an existing attribute]
     expected: FAIL
 
-  [add on DOMTokenList must not enqueue an attributeChanged reaction when adding a value to an unobserved attribute]
-    expected: FAIL
-
   [add on DOMTokenList must enqueue exactly one attributeChanged reaction when adding multiple values to an attribute]
     expected: FAIL
 
@@ -24,9 +18,6 @@
   [remove on DOMTokenList must enqueue an attributeChanged reaction even when removing a non-existent value from an attribute]
     expected: FAIL
 
-  [remove on DOMTokenList must not enqueue an attributeChanged reaction when removing a value from an unobserved attribute]
-    expected: FAIL
-
   [toggle on DOMTokenList must enqueue an attributeChanged reaction when adding a value to an attribute]
     expected: FAIL
 
@@ -39,21 +30,12 @@
   [replace on DOMTokenList must not enqueue an attributeChanged reaction when the token to replace does not exist in the attribute]
     expected: FAIL
 
-  [replace on DOMTokenList must not enqueue an attributeChanged reaction when replacing a value in an unobserved attribute]
-    expected: FAIL
-
   [the stringifier of DOMTokenList must enqueue an attributeChanged reaction when adding an observed attribute]
     expected: FAIL
 
-  [the stringifier of DOMTokenList must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [the stringifier of DOMTokenList must enqueue an attributeChanged reaction when mutating the value of an observed attribute]
     expected: FAIL
 
-  [the stringifier of DOMTokenList must not enqueue an attributeChanged reaction when mutating the value of an unobserved attribute]
-    expected: FAIL
-
   [the stringifier of DOMTokenList must enqueue an attributeChanged reaction when the setter is called with the original value of the attribute]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/reactions/Document.html.ini b/tests/wpt/metadata/custom-elements/reactions/Document.html.ini
index a31a31b0d2e..07e68da69b7 100644
--- a/tests/wpt/metadata/custom-elements/reactions/Document.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/Document.html.ini
@@ -1,11 +1,5 @@
 [Document.html]
   type: testharness
-  [importNode on Document must not construct a new custom element when importing a custom element into a window-less document]
-    expected: FAIL
-
-  [importNode on Document must construct a new custom element when importing a custom element from a template]
-    expected: FAIL
-
   [adoptNode on Document must enqueue an adopted reaction when importing a custom element]
     expected: FAIL
 
@@ -36,3 +30,6 @@
   [writeln on Document must enqueue connectedCallback after constructing a custom element]
     expected: FAIL
 
+  [importNode on Document must construct a new custom element when importing a custom element from a template]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/custom-elements/reactions/Element.html.ini b/tests/wpt/metadata/custom-elements/reactions/Element.html.ini
index 9407ce44d26..fa721851548 100644
--- a/tests/wpt/metadata/custom-elements/reactions/Element.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/Element.html.ini
@@ -21,78 +21,36 @@
   [setAttribute on Element must enqueue an attributeChanged reaction when adding an attribute]
     expected: FAIL
 
-  [setAttribute on Element must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [setAttribute on Element must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [setAttribute on Element must enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
   [setAttributeNS on Element must enqueue an attributeChanged reaction when adding an attribute]
     expected: FAIL
 
-  [setAttributeNS on Element must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [setAttributeNS on Element must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [setAttributeNS on Element must enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
-  [removeAttribute on Element must not enqueue an attributeChanged reaction when removing an unobserved attribute]
-    expected: FAIL
-
   [removeAttribute on Element must enqueue an attributeChanged reaction when removing an existing attribute]
     expected: FAIL
 
-  [removeAttribute on Element must not enqueue an attributeChanged reaction when removing an existing unobserved attribute]
-    expected: FAIL
-
-  [removeAttributeNS on Element must not enqueue an attributeChanged reaction when removing an unobserved attribute]
-    expected: FAIL
-
   [removeAttributeNS on Element must enqueue an attributeChanged reaction when removing an existing attribute]
     expected: FAIL
 
-  [removeAttributeNS on Element must not enqueue an attributeChanged reaction when removing an existing unobserved attribute]
-    expected: FAIL
-
   [setAttributeNode on Element must enqueue an attributeChanged reaction when adding an attribute]
     expected: FAIL
 
-  [setAttributeNode on Element must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [setAttributeNode on Element must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [setAttributeNode on Element must enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
   [setAttributeNodeNS on Element must enqueue an attributeChanged reaction when adding an attribute]
     expected: FAIL
 
-  [setAttributeNodeNS on Element must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [setAttributeNodeNS on Element must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [setAttributeNodeNS on Element must enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
-  [removeAttributeNode on Element must not enqueue an attributeChanged reaction when removing an unobserved attribute]
-    expected: FAIL
-
   [removeAttributeNode on Element must enqueue an attributeChanged reaction when removing an existing attribute]
     expected: FAIL
 
-  [removeAttributeNode on Element must not enqueue an attributeChanged reaction when removing an existing unobserved attribute]
-    expected: FAIL
-
   [insertAdjacentElement on Element must enqueue a connected reaction]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/reactions/NamedNodeMap.html.ini b/tests/wpt/metadata/custom-elements/reactions/NamedNodeMap.html.ini
index 9361e3c2d0f..f1ffa8a7189 100644
--- a/tests/wpt/metadata/custom-elements/reactions/NamedNodeMap.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/NamedNodeMap.html.ini
@@ -3,42 +3,18 @@
   [setNamedItem on NamedNodeMap must enqueue an attributeChanged reaction when adding an attribute]
     expected: FAIL
 
-  [setNamedItem on NamedNodeMap must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [setNamedItem on NamedNodeMap must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [setNamedItem on NamedNodeMap must enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
   [setNamedItemNS on NamedNodeMap must enqueue an attributeChanged reaction when adding an attribute]
     expected: FAIL
 
-  [setNamedItemNS on NamedNodeMap must not enqueue an attributeChanged reaction when adding an unobserved attribute]
-    expected: FAIL
-
   [setNamedItemNS on NamedNodeMap must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [setNamedItemNS on NamedNodeMap must enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
-  [removeNamedItem on NamedNodeMap must not enqueue an attributeChanged reaction when removing an unobserved attribute]
-    expected: FAIL
-
   [removeNamedItem on NamedNodeMap must enqueue an attributeChanged reaction when removing an existing attribute]
     expected: FAIL
 
-  [removeNamedItem on NamedNodeMap must not enqueue an attributeChanged reaction when removing an existing unobserved attribute]
-    expected: FAIL
-
-  [removeNamedItemNS on NamedNodeMap must not enqueue an attributeChanged reaction when removing an unobserved attribute]
-    expected: FAIL
-
   [removeNamedItemNS on NamedNodeMap must enqueue an attributeChanged reaction when removing an existing attribute]
     expected: FAIL
 
-  [removeNamedItemNS on NamedNodeMap must not enqueue an attributeChanged reaction when removing an existing unobserved attribute]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/custom-elements/reactions/Node.html.ini b/tests/wpt/metadata/custom-elements/reactions/Node.html.ini
index 87acc9aa613..a5be84ba635 100644
--- a/tests/wpt/metadata/custom-elements/reactions/Node.html.ini
+++ b/tests/wpt/metadata/custom-elements/reactions/Node.html.ini
@@ -3,15 +3,9 @@
   [nodeValue on Node must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [nodeValue on Node must not enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
   [textContent on Node must enqueue an attributeChanged reaction when replacing an existing attribute]
     expected: FAIL
 
-  [textContent on Node must not enqueue an attributeChanged reaction when replacing an existing unobserved attribute]
-    expected: FAIL
-
   [cloneNode on Node must enqueue an attributeChanged reaction when cloning an element with an observed attribute]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/upgrading.html.ini b/tests/wpt/metadata/custom-elements/upgrading.html.ini
index 9a070a44860..269f72b5603 100644
--- a/tests/wpt/metadata/custom-elements/upgrading.html.ini
+++ b/tests/wpt/metadata/custom-elements/upgrading.html.ini
@@ -36,9 +36,6 @@
   [Creating an element in an HTML document fetched by XHR and adopting back to a document with browsing context must enqueue a custom element upgrade reaction]
     expected: FAIL
 
-  [Creating an element in the document of an iframe must enqueue a custom element upgrade reaction if there is a matching definition]
-    expected: FAIL
-
   ["define" in the document of an iframe must not enqueue a custom element upgrade reaction on a disconnected unresolved custom element]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/custom-elements/upgrading/Node-cloneNode.html.ini b/tests/wpt/metadata/custom-elements/upgrading/Node-cloneNode.html.ini
index 3214c6fd3b0..36527742adc 100644
--- a/tests/wpt/metadata/custom-elements/upgrading/Node-cloneNode.html.ini
+++ b/tests/wpt/metadata/custom-elements/upgrading/Node-cloneNode.html.ini
@@ -1,14 +1,5 @@
 [Node-cloneNode.html]
   type: testharness
-  [Node.prototype.cloneNode(false) must be able to clone a custom element]
-    expected: FAIL
-
-  [Node.prototype.cloneNode(false) must be able to clone a custom element inside an iframe]
-    expected: FAIL
-
-  [Node.prototype.cloneNode(true) must be able to clone a descendent custom element]
-    expected: FAIL
-
   [Node.prototype.cloneNode(true) must set parentNode, previousSibling, and nextSibling before upgrading custom elements]
     expected: FAIL
 
@@ -24,3 +15,12 @@
   [Inserting an element must not try to upgrade a custom element when it had already failed to upgrade once]
     expected: FAIL
 
+  [Node.prototype.cloneNode(false) must be able to clone a custom element]
+    expected: FAIL
+
+  [Node.prototype.cloneNode(false) must be able to clone a custom element inside an iframe]
+    expected: FAIL
+
+  [Node.prototype.cloneNode(true) must be able to clone a descendent custom element]
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/html/browsers/browsing-the-web/scroll-to-fragid/003.html.ini b/tests/wpt/metadata/html/browsers/browsing-the-web/scroll-to-fragid/003.html.ini
index ae630f04075..23e4a811c3d 100644
--- a/tests/wpt/metadata/html/browsers/browsing-the-web/scroll-to-fragid/003.html.ini
+++ b/tests/wpt/metadata/html/browsers/browsing-the-web/scroll-to-fragid/003.html.ini
@@ -1,4 +1,5 @@
 [003.html]
   type: testharness
   [Fragment Navigation: Updating scroll position]
-    expected: FAIL
\ No newline at end of file
+    expected: FAIL
+
diff --git a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe-append-to-child-document.html.ini b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe-append-to-child-document.html.ini
index d73bfee8141..af1f00202d3 100644
--- a/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe-append-to-child-document.html.ini
+++ b/tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe-append-to-child-document.html.ini
@@ -3,3 +3,4 @@
   [Append iframe element to its own child document]
     bug: https://github.com/servo/servo/issues/17479
     expected: FAIL
+
diff --git a/tests/wpt/web-platform-tests/custom-elements/Document-createElement.html b/tests/wpt/web-platform-tests/custom-elements/Document-createElement.html
index f8b3a94c00d..e446c507ca8 100644
--- a/tests/wpt/web-platform-tests/custom-elements/Document-createElement.html
+++ b/tests/wpt/web-platform-tests/custom-elements/Document-createElement.html
@@ -8,7 +8,7 @@
 <link rel="help" content="https://dom.spec.whatwg.org/#concept-create-element">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
-<script src="resources/custom-elements-helper.js"></script>
+<script src="resources/custom-elements-helpers.js"></script>
 </head>
 <body>
 <div id="log"></div>