/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

//! The core DOM types. Defines the basic DOM hierarchy as well as all the HTML elements.

use app_units::Au;
use devtools_traits::NodeInfo;
use document_loader::DocumentLoader;
use dom::bindings::cell::DomRefCell;
use dom::bindings::codegen::Bindings::CharacterDataBinding::CharacterDataMethods;
use dom::bindings::codegen::Bindings::DocumentBinding::DocumentMethods;
use dom::bindings::codegen::Bindings::ElementBinding::ElementMethods;
use dom::bindings::codegen::Bindings::HTMLCollectionBinding::HTMLCollectionMethods;
use dom::bindings::codegen::Bindings::NodeBinding::{NodeConstants, NodeMethods};
use dom::bindings::codegen::Bindings::NodeListBinding::NodeListMethods;
use dom::bindings::codegen::Bindings::ProcessingInstructionBinding::ProcessingInstructionMethods;
use dom::bindings::codegen::Bindings::WindowBinding::WindowMethods;
use dom::bindings::codegen::UnionTypes::NodeOrString;
use dom::bindings::conversions::{self, DerivedFrom};
use dom::bindings::error::{Error, ErrorResult, Fallible};
use dom::bindings::inheritance::{Castable, CharacterDataTypeId, ElementTypeId};
use dom::bindings::inheritance::{EventTargetTypeId, HTMLElementTypeId, NodeTypeId};
use dom::bindings::inheritance::{SVGElementTypeId, SVGGraphicsElementTypeId};
use dom::bindings::reflector::{DomObject, reflect_dom_object};
use dom::bindings::root::{Dom, DomRoot, LayoutDom, MutNullableDom, RootedReference};
use dom::bindings::str::{DOMString, USVString};
use dom::bindings::xmlname::namespace_from_domstring;
use dom::characterdata::{CharacterData, LayoutCharacterDataHelpers};
use dom::cssstylesheet::CSSStyleSheet;
use dom::customelementregistry::{CallbackReaction, try_upgrade_element};
use dom::document::{Document, DocumentSource, HasBrowsingContext, IsHTMLDocument};
use dom::documentfragment::DocumentFragment;
use dom::documenttype::DocumentType;
use dom::element::{CustomElementCreationMode, Element, ElementCreator};
use dom::eventtarget::EventTarget;
use dom::globalscope::GlobalScope;
use dom::htmlbodyelement::HTMLBodyElement;
use dom::htmlcanvaselement::{HTMLCanvasElement, LayoutHTMLCanvasElementHelpers};
use dom::htmlcollection::HTMLCollection;
use dom::htmlelement::HTMLElement;
use dom::htmliframeelement::{HTMLIFrameElement, HTMLIFrameElementLayoutMethods};
use dom::htmlimageelement::{HTMLImageElement, LayoutHTMLImageElementHelpers};
use dom::htmlinputelement::{HTMLInputElement, LayoutHTMLInputElementHelpers};
use dom::htmllinkelement::HTMLLinkElement;
use dom::htmlmediaelement::{HTMLMediaElement, LayoutHTMLMediaElementHelpers};
use dom::htmlmetaelement::HTMLMetaElement;
use dom::htmlstyleelement::HTMLStyleElement;
use dom::htmltextareaelement::{HTMLTextAreaElement, LayoutHTMLTextAreaElementHelpers};
use dom::mutationobserver::{Mutation, MutationObserver, RegisteredObserver};
use dom::nodelist::NodeList;
use dom::processinginstruction::ProcessingInstruction;
use dom::range::WeakRangeVec;
use dom::svgsvgelement::{SVGSVGElement, LayoutSVGSVGElementHelpers};
use dom::text::Text;
use dom::virtualmethods::{VirtualMethods, vtable_for};
use dom::window::Window;
use dom_struct::dom_struct;
use euclid::{Point2D, Vector2D, Rect, Size2D};
use html5ever::{Prefix, Namespace, QualName};
use js::jsapi::{JSContext, JSObject, JSRuntime};
use libc::{self, c_void, uintptr_t};
use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
use msg::constellation_msg::{BrowsingContextId, PipelineId};
use net_traits::image::base::{Image, ImageMetadata};
use ref_slice::ref_slice;
use script_layout_interface::{HTMLCanvasData, HTMLMediaData, LayoutElementType, LayoutNodeType};
use script_layout_interface::{OpaqueStyleAndLayoutData, SVGSVGData, TrustedNodeAddress};
use script_layout_interface::message::Msg;
use script_thread::ScriptThread;
use script_traits::DocumentActivity;
use script_traits::UntrustedNodeAddress;
use selectors::matching::{matches_selector_list, MatchingContext, MatchingMode};
use selectors::parser::SelectorList;
use servo_arc::Arc;
use servo_url::ServoUrl;
use smallvec::SmallVec;
use std::borrow::ToOwned;
use std::cell::{Cell, UnsafeCell, RefMut};
use std::cmp;
use std::default::Default;
use std::iter;
use std::mem;
use std::ops::Range;
use std::sync::Arc as StdArc;
use style::context::QuirksMode;
use style::dom::OpaqueNode;
use style::selector_parser::{SelectorImpl, SelectorParser};
use style::stylesheets::Stylesheet;
use style::thread_state;
use uuid::Uuid;

//
// The basic Node structure
//

/// An HTML node.
#[dom_struct]
pub struct Node {
    /// The JavaScript reflector for this node.
    eventtarget: EventTarget,

    /// The parent of this node.
    parent_node: MutNullableDom<Node>,

    /// The first child of this node.
    first_child: MutNullableDom<Node>,

    /// The last child of this node.
    last_child: MutNullableDom<Node>,

    /// The next sibling of this node.
    next_sibling: MutNullableDom<Node>,

    /// The previous sibling of this node.
    prev_sibling: MutNullableDom<Node>,

    /// The document that this node belongs to.
    owner_doc: MutNullableDom<Document>,

    /// The live list of children return by .childNodes.
    child_list: MutNullableDom<NodeList>,

    /// The live count of children of this node.
    children_count: Cell<u32>,

    /// A bitfield of flags for node items.
    flags: Cell<NodeFlags>,

    /// The maximum version of any inclusive descendant of this node.
    inclusive_descendants_version: Cell<u64>,

    /// A vector of weak references to Range instances of which the start
    /// or end containers are this node. No range should ever be found
    /// twice in this vector, even if both the start and end containers
    /// are this node.
    ranges: WeakRangeVec,

    /// Style+Layout information. Only the layout thread may touch this data.
    ///
    /// Must be sent back to the layout thread to be destroyed when this
    /// node is finalized.
    style_and_layout_data: Cell<Option<OpaqueStyleAndLayoutData>>,

    /// Registered observers for this node.
    mutation_observers: DomRefCell<Vec<RegisteredObserver>>,

    unique_id: UniqueId,
}

bitflags! {
    #[doc = "Flags for node items."]
    #[derive(JSTraceable, MallocSizeOf)]
    pub struct NodeFlags: u16 {
        #[doc = "Specifies whether this node is in a document."]
        const IS_IN_DOC = 1 << 0;

        #[doc = "Specifies whether this node needs style recalc on next reflow."]
        const HAS_DIRTY_DESCENDANTS = 1 << 1;
        // TODO: find a better place to keep this (#4105)
        // https://critic.hoppipolla.co.uk/showcomment?chain=8873
        // Perhaps using a Set in Document?
        #[doc = "Specifies whether or not there is an authentic click in progress on \
                 this element."]
        const CLICK_IN_PROGRESS = 1 << 2;
        #[doc = "Specifies whether this node is focusable and whether it is supposed \
                 to be reachable with using sequential focus navigation."]
        const SEQUENTIALLY_FOCUSABLE = 1 << 3;

        // There are two free bits here.

        #[doc = "Specifies whether the parser has set an associated form owner for \
                 this element. Only applicable for form-associatable elements."]
        const PARSER_ASSOCIATED_FORM_OWNER = 1 << 6;

        /// Whether this element has a snapshot stored due to a style or
        /// attribute change.
        ///
        /// See the `style::restyle_hints` module.
        const HAS_SNAPSHOT = 1 << 7;

        /// Whether this element has already handled the stored snapshot.
        const HANDLED_SNAPSHOT = 1 << 8;
    }
}

impl NodeFlags {
    pub fn new() -> NodeFlags {
        NodeFlags::empty()
    }
}

impl Drop for Node {
    #[allow(unsafe_code)]
    fn drop(&mut self) {
        self.style_and_layout_data.get().map(|d| self.dispose(d));
    }
}

/// suppress observers flag
/// <https://dom.spec.whatwg.org/#concept-node-insert>
/// <https://dom.spec.whatwg.org/#concept-node-remove>
#[derive(Clone, Copy, MallocSizeOf)]
enum SuppressObserver {
    Suppressed,
    Unsuppressed,
}

impl Node {
    /// Sends the style and layout data, if any, back to the layout thread to be destroyed.
    pub fn dispose(&self, data: OpaqueStyleAndLayoutData) {
        debug_assert!(thread_state::get().is_script());
        let win = window_from_node(self);
        self.style_and_layout_data.set(None);
        if win
            .layout_chan()
            .send(Msg::ReapStyleAndLayoutData(data))
            .is_err()
        {
            warn!("layout thread unreachable - leaking layout data");
        }
    }

    /// Adds a new child to the end of this node's list of children.
    ///
    /// Fails unless `new_child` is disconnected from the tree.
    fn add_child(&self, new_child: &Node, before: Option<&Node>) {
        assert!(new_child.parent_node.get().is_none());
        assert!(new_child.prev_sibling.get().is_none());
        assert!(new_child.next_sibling.get().is_none());
        match before {
            Some(ref before) => {
                assert!(before.parent_node.get().r() == Some(self));
                let prev_sibling = before.GetPreviousSibling();
                match prev_sibling {
                    None => {
                        assert!(Some(*before) == self.first_child.get().r());
                        self.first_child.set(Some(new_child));
                    },
                    Some(ref prev_sibling) => {
                        prev_sibling.next_sibling.set(Some(new_child));
                        new_child.prev_sibling.set(Some(&prev_sibling));
                    },
                }
                before.prev_sibling.set(Some(new_child));
                new_child.next_sibling.set(Some(before));
            },
            None => {
                let last_child = self.GetLastChild();
                match last_child {
                    None => self.first_child.set(Some(new_child)),
                    Some(ref last_child) => {
                        assert!(last_child.next_sibling.get().is_none());
                        last_child.next_sibling.set(Some(new_child));
                        new_child.prev_sibling.set(Some(&last_child));
                    },
                }

                self.last_child.set(Some(new_child));
            },
        }

        new_child.parent_node.set(Some(self));
        self.children_count.set(self.children_count.get() + 1);

        let parent_in_doc = self.is_in_doc();
        for node in new_child.traverse_preorder() {
            node.set_flag(NodeFlags::IS_IN_DOC, parent_in_doc);
            // Out-of-document elements never have the descendants flag set.
            debug_assert!(!node.get_flag(NodeFlags::HAS_DIRTY_DESCENDANTS));
            vtable_for(&&*node).bind_to_tree(parent_in_doc);
        }
    }

    /// Removes the given child from this node's list of children.
    ///
    /// Fails unless `child` is a child of this node.
    fn remove_child(&self, child: &Node, cached_index: Option<u32>) {
        assert!(child.parent_node.get().r() == Some(self));
        let prev_sibling = child.GetPreviousSibling();
        match prev_sibling {
            None => {
                self.first_child.set(child.next_sibling.get().r());
            },
            Some(ref prev_sibling) => {
                prev_sibling.next_sibling.set(child.next_sibling.get().r());
            },
        }
        let next_sibling = child.GetNextSibling();
        match next_sibling {
            None => {
                self.last_child.set(child.prev_sibling.get().r());
            },
            Some(ref next_sibling) => {
                next_sibling.prev_sibling.set(child.prev_sibling.get().r());
            },
        }

        let context = UnbindContext::new(self, prev_sibling.r(), next_sibling.r(), cached_index);

        child.prev_sibling.set(None);
        child.next_sibling.set(None);
        child.parent_node.set(None);
        self.children_count.set(self.children_count.get() - 1);

        for node in child.traverse_preorder() {
            // Out-of-document elements never have the descendants flag set.
            node.set_flag(
                NodeFlags::IS_IN_DOC |
                    NodeFlags::HAS_DIRTY_DESCENDANTS |
                    NodeFlags::HAS_SNAPSHOT |
                    NodeFlags::HANDLED_SNAPSHOT,
                false,
            );
        }
        for node in child.traverse_preorder() {
            // This needs to be in its own loop, because unbind_from_tree may
            // rely on the state of IS_IN_DOC of the context node's descendants,
            // e.g. when removing a <form>.
            vtable_for(&&*node).unbind_from_tree(&context);
            node.style_and_layout_data.get().map(|d| node.dispose(d));
            // https://dom.spec.whatwg.org/#concept-node-remove step 14
            if let Some(element) = node.as_custom_element() {
                ScriptThread::enqueue_callback_reaction(
                    &*element,
                    CallbackReaction::Disconnected,
                    None,
                );
            }
        }
    }

    pub fn to_untrusted_node_address(&self) -> UntrustedNodeAddress {
        UntrustedNodeAddress(self.reflector().get_jsobject().get() as *const c_void)
    }

    pub fn as_custom_element(&self) -> Option<DomRoot<Element>> {
        self.downcast::<Element>().and_then(|element| {
            if element.get_custom_element_definition().is_some() {
                Some(DomRoot::from_ref(element))
            } else {
                None
            }
        })
    }
}

pub struct QuerySelectorIterator {
    selectors: SelectorList<SelectorImpl>,
    iterator: TreeIterator,
}

impl<'a> QuerySelectorIterator {
    fn new(iter: TreeIterator, selectors: SelectorList<SelectorImpl>) -> QuerySelectorIterator {
        QuerySelectorIterator {
            selectors: selectors,
            iterator: iter,
        }
    }
}

impl<'a> Iterator for QuerySelectorIterator {
    type Item = DomRoot<Node>;

    fn next(&mut self) -> Option<DomRoot<Node>> {
        let selectors = &self.selectors;

        self.iterator
            .by_ref()
            .filter_map(|node| {
                // TODO(cgaebel): Is it worth it to build a bloom filter here
                // (instead of passing `None`)? Probably.
                //
                // FIXME(bholley): Consider an nth-index cache here.
                let mut ctx = MatchingContext::new(
                    MatchingMode::Normal,
                    None,
                    None,
                    node.owner_doc().quirks_mode(),
                );
                if let Some(element) = DomRoot::downcast(node) {
                    if matches_selector_list(selectors, &element, &mut ctx) {
                        return Some(DomRoot::upcast(element));
                    }
                }
                None
            }).next()
    }
}

impl Node {
    pub fn teardown(&self) {
        self.style_and_layout_data.get().map(|d| self.dispose(d));
        for kid in self.children() {
            kid.teardown();
        }
    }

    /// Returns true if this node is before `other` in the same connected DOM
    /// tree.
    pub fn is_before(&self, other: &Node) -> bool {
        let cmp = other.CompareDocumentPosition(self);
        if cmp & NodeConstants::DOCUMENT_POSITION_DISCONNECTED != 0 {
            return false;
        }

        cmp & NodeConstants::DOCUMENT_POSITION_PRECEDING != 0
    }

    /// Return all registered mutation observers for this node.
    pub fn registered_mutation_observers(&self) -> RefMut<Vec<RegisteredObserver>> {
        self.mutation_observers.borrow_mut()
    }

    /// Removes the mutation observer for a given node.
    pub fn remove_mutation_observer(&self, observer: &MutationObserver) {
        self.mutation_observers
            .borrow_mut()
            .retain(|reg_obs| &*reg_obs.observer != observer)
    }

    /// Dumps the subtree rooted at this node, for debugging.
    pub fn dump(&self) {
        self.dump_indent(0);
    }

    /// Dumps the node tree, for debugging, with indentation.
    pub fn dump_indent(&self, indent: u32) {
        let mut s = String::new();
        for _ in 0..indent {
            s.push_str("    ");
        }

        s.push_str(&*self.debug_str());
        debug!("{:?}", s);

        // FIXME: this should have a pure version?
        for kid in self.children() {
            kid.dump_indent(indent + 1)
        }
    }

    /// Returns a string that describes this node.
    pub fn debug_str(&self) -> String {
        format!("{:?}", self.type_id())
    }

    pub fn is_in_doc(&self) -> bool {
        self.flags.get().contains(NodeFlags::IS_IN_DOC)
    }

    /// Returns the type ID of this node.
    pub fn type_id(&self) -> NodeTypeId {
        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() {
            NodeTypeId::DocumentType => 0,
            NodeTypeId::CharacterData(_) => self.downcast::<CharacterData>().unwrap().Length(),
            _ => self.children_count(),
        }
    }

    // https://dom.spec.whatwg.org/#concept-tree-index
    pub fn index(&self) -> u32 {
        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()
    }

    pub fn ranges(&self) -> &WeakRangeVec {
        &self.ranges
    }

    #[inline]
    pub fn is_doctype(&self) -> bool {
        self.type_id() == NodeTypeId::DocumentType
    }

    pub fn get_flag(&self, flag: NodeFlags) -> bool {
        self.flags.get().contains(flag)
    }

    pub fn set_flag(&self, flag: NodeFlags, value: bool) {
        let mut flags = self.flags.get();

        if value {
            flags.insert(flag);
        } else {
            flags.remove(flag);
        }

        self.flags.set(flags);
    }

    // FIXME(emilio): This and the function below should move to Element.
    pub fn note_dirty_descendants(&self) {
        debug_assert!(self.is_in_doc());

        for ancestor in self.inclusive_ancestors() {
            if ancestor.get_flag(NodeFlags::HAS_DIRTY_DESCENDANTS) {
                return;
            }

            ancestor.set_flag(NodeFlags::HAS_DIRTY_DESCENDANTS, true);
        }
    }

    pub fn has_dirty_descendants(&self) -> bool {
        self.get_flag(NodeFlags::HAS_DIRTY_DESCENDANTS)
    }

    pub fn rev_version(&self) {
        // The new version counter is 1 plus the max of the node's current version counter,
        // its descendants version, and the document's version. Normally, this will just be
        // the document's version, but we do have to deal with the case where the node has moved
        // document, so may have a higher version count than its owning document.
        let doc: DomRoot<Node> = DomRoot::upcast(self.owner_doc());
        let version = cmp::max(
            self.inclusive_descendants_version(),
            doc.inclusive_descendants_version(),
        ) + 1;
        for ancestor in self.inclusive_ancestors() {
            ancestor.inclusive_descendants_version.set(version);
        }
        doc.inclusive_descendants_version.set(version);
    }

    pub fn dirty(&self, damage: NodeDamage) {
        self.rev_version();
        if !self.is_in_doc() {
            return;
        }

        match self.type_id() {
            NodeTypeId::CharacterData(CharacterDataTypeId::Text) => self
                .parent_node
                .get()
                .unwrap()
                .downcast::<Element>()
                .unwrap()
                .restyle(damage),
            NodeTypeId::Element(_) => self.downcast::<Element>().unwrap().restyle(damage),
            _ => {},
        };
    }

    /// The maximum version number of this node's descendants, including itself
    pub fn inclusive_descendants_version(&self) -> u64 {
        self.inclusive_descendants_version.get()
    }

    /// Iterates over this node and all its descendants, in preorder.
    pub fn traverse_preorder(&self) -> TreeIterator {
        TreeIterator::new(self)
    }

    pub fn inclusively_following_siblings(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: Some(DomRoot::from_ref(self)),
            next_node: |n| n.GetNextSibling(),
        }
    }

    pub fn inclusively_preceding_siblings(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: Some(DomRoot::from_ref(self)),
            next_node: |n| n.GetPreviousSibling(),
        }
    }

    pub fn is_inclusive_ancestor_of(&self, parent: &Node) -> bool {
        self == parent || self.is_ancestor_of(parent)
    }

    pub fn is_ancestor_of(&self, parent: &Node) -> bool {
        parent.ancestors().any(|ancestor| &*ancestor == self)
    }

    pub fn following_siblings(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: self.GetNextSibling(),
            next_node: |n| n.GetNextSibling(),
        }
    }

    pub fn preceding_siblings(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: self.GetPreviousSibling(),
            next_node: |n| n.GetPreviousSibling(),
        }
    }

    pub fn following_nodes(&self, root: &Node) -> FollowingNodeIterator {
        FollowingNodeIterator {
            current: Some(DomRoot::from_ref(self)),
            root: DomRoot::from_ref(root),
        }
    }

    pub fn preceding_nodes(&self, root: &Node) -> PrecedingNodeIterator {
        PrecedingNodeIterator {
            current: Some(DomRoot::from_ref(self)),
            root: DomRoot::from_ref(root),
        }
    }

    pub fn descending_last_children(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: self.GetLastChild(),
            next_node: |n| n.GetLastChild(),
        }
    }

    pub fn is_parent_of(&self, child: &Node) -> bool {
        child
            .parent_node
            .get()
            .map_or(false, |parent| &*parent == self)
    }

    pub fn to_trusted_node_address(&self) -> TrustedNodeAddress {
        TrustedNodeAddress(&*self as *const Node as *const libc::c_void)
    }

    /// Returns the rendered bounding content box if the element is rendered,
    /// and none otherwise.
    pub fn bounding_content_box(&self) -> Option<Rect<Au>> {
        window_from_node(self).content_box_query(self.to_trusted_node_address())
    }

    pub fn bounding_content_box_or_zero(&self) -> Rect<Au> {
        self.bounding_content_box().unwrap_or_else(Rect::zero)
    }

    pub fn content_boxes(&self) -> Vec<Rect<Au>> {
        window_from_node(self).content_boxes_query(self.to_trusted_node_address())
    }

    pub fn client_rect(&self) -> Rect<i32> {
        window_from_node(self).client_rect_query(self.to_trusted_node_address())
    }

    // https://drafts.csswg.org/cssom-view/#dom-element-scrollwidth
    // https://drafts.csswg.org/cssom-view/#dom-element-scrollheight
    pub fn scroll_area(&self) -> Rect<i32> {
        // Step 1
        let document = self.owner_doc();
        // Step 3
        let window = document.window();

        let html_element = document.GetDocumentElement();

        let is_body_element = self
            .downcast::<HTMLBodyElement>()
            .map_or(false, |e| e.is_the_html_body_element());

        let scroll_area = window.scroll_area_query(self.to_trusted_node_address());

        match (
            document != window.Document(),
            is_body_element,
            document.quirks_mode(),
            html_element.r() == self.downcast::<Element>(),
        ) {
            // Step 2 && Step 5
            (true, _, _, _) | (_, false, QuirksMode::Quirks, true) => Rect::zero(),
            // Step 6 && Step 7
            (false, false, _, true) | (false, true, QuirksMode::Quirks, _) => Rect::new(
                Point2D::new(window.ScrollX(), window.ScrollY()),
                Size2D::new(
                    cmp::max(window.InnerWidth(), scroll_area.size.width),
                    cmp::max(window.InnerHeight(), scroll_area.size.height),
                ),
            ),
            // Step 9
            _ => scroll_area,
        }
    }

    pub fn scroll_offset(&self) -> Vector2D<f32> {
        let document = self.owner_doc();
        let window = document.window();
        window.scroll_offset_query(self)
    }

    // https://dom.spec.whatwg.org/#dom-childnode-before
    pub fn before(&self, nodes: Vec<NodeOrString>) -> ErrorResult {
        // Step 1.
        let parent = &self.parent_node;

        // Step 2.
        let parent = match parent.get() {
            None => return Ok(()),
            Some(parent) => parent,
        };

        // Step 3.
        let viable_previous_sibling = first_node_not_in(self.preceding_siblings(), &nodes);

        // Step 4.
        let node = self.owner_doc().node_from_nodes_and_strings(nodes)?;

        // Step 5.
        let viable_previous_sibling = match viable_previous_sibling {
            Some(ref viable_previous_sibling) => viable_previous_sibling.next_sibling.get(),
            None => parent.first_child.get(),
        };

        // Step 6.
        Node::pre_insert(&node, &parent, viable_previous_sibling.r())?;

        Ok(())
    }

    // https://dom.spec.whatwg.org/#dom-childnode-after
    pub fn after(&self, nodes: Vec<NodeOrString>) -> ErrorResult {
        // Step 1.
        let parent = &self.parent_node;

        // Step 2.
        let parent = match parent.get() {
            None => return Ok(()),
            Some(parent) => parent,
        };

        // Step 3.
        let viable_next_sibling = first_node_not_in(self.following_siblings(), &nodes);

        // Step 4.
        let node = self.owner_doc().node_from_nodes_and_strings(nodes)?;

        // Step 5.
        Node::pre_insert(&node, &parent, viable_next_sibling.r())?;

        Ok(())
    }

    // https://dom.spec.whatwg.org/#dom-childnode-replacewith
    pub fn replace_with(&self, nodes: Vec<NodeOrString>) -> ErrorResult {
        // Step 1.
        let parent = if let Some(parent) = self.GetParentNode() {
            parent
        } else {
            // Step 2.
            return Ok(());
        };
        // Step 3.
        let viable_next_sibling = first_node_not_in(self.following_siblings(), &nodes);
        // Step 4.
        let node = self.owner_doc().node_from_nodes_and_strings(nodes)?;
        if self.parent_node == Some(&*parent) {
            // Step 5.
            parent.ReplaceChild(&node, self)?;
        } else {
            // Step 6.
            Node::pre_insert(&node, &parent, viable_next_sibling.r())?;
        }
        Ok(())
    }

    // https://dom.spec.whatwg.org/#dom-parentnode-prepend
    pub fn prepend(&self, nodes: Vec<NodeOrString>) -> ErrorResult {
        // Step 1.
        let doc = self.owner_doc();
        let node = doc.node_from_nodes_and_strings(nodes)?;
        // Step 2.
        let first_child = self.first_child.get();
        Node::pre_insert(&node, self, first_child.r()).map(|_| ())
    }

    // https://dom.spec.whatwg.org/#dom-parentnode-append
    pub fn append(&self, nodes: Vec<NodeOrString>) -> ErrorResult {
        // Step 1.
        let doc = self.owner_doc();
        let node = doc.node_from_nodes_and_strings(nodes)?;
        // Step 2.
        self.AppendChild(&node).map(|_| ())
    }

    // https://dom.spec.whatwg.org/#dom-parentnode-queryselector
    pub fn query_selector(&self, selectors: DOMString) -> Fallible<Option<DomRoot<Element>>> {
        // Step 1.
        match SelectorParser::parse_author_origin_no_namespace(&selectors) {
            // Step 2.
            Err(_) => Err(Error::Syntax),
            // Step 3.
            Ok(selectors) => {
                // FIXME(bholley): Consider an nth-index cache here.
                let mut ctx = MatchingContext::new(
                    MatchingMode::Normal,
                    None,
                    None,
                    self.owner_doc().quirks_mode(),
                );
                Ok(self
                    .traverse_preorder()
                    .filter_map(DomRoot::downcast)
                    .find(|element| matches_selector_list(&selectors, element, &mut ctx)))
            },
        }
    }

    /// <https://dom.spec.whatwg.org/#scope-match-a-selectors-string>
    /// Get an iterator over all nodes which match a set of selectors
    /// Be careful not to do anything which may manipulate the DOM tree
    /// whilst iterating, otherwise the iterator may be invalidated.
    pub fn query_selector_iter(&self, selectors: DOMString) -> Fallible<QuerySelectorIterator> {
        // Step 1.
        match SelectorParser::parse_author_origin_no_namespace(&selectors) {
            // Step 2.
            Err(_) => Err(Error::Syntax),
            // Step 3.
            Ok(selectors) => {
                let mut descendants = self.traverse_preorder();
                // Skip the root of the tree.
                assert!(&*descendants.next().unwrap() == self);
                Ok(QuerySelectorIterator::new(descendants, selectors))
            },
        }
    }

    // https://dom.spec.whatwg.org/#dom-parentnode-queryselectorall
    #[allow(unsafe_code)]
    pub fn query_selector_all(&self, selectors: DOMString) -> Fallible<DomRoot<NodeList>> {
        let window = window_from_node(self);
        let iter = self.query_selector_iter(selectors)?;
        Ok(NodeList::new_simple_list(&window, iter))
    }

    pub fn ancestors(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: self.GetParentNode(),
            next_node: |n| n.GetParentNode(),
        }
    }

    pub fn inclusive_ancestors(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: Some(DomRoot::from_ref(self)),
            next_node: |n| n.GetParentNode(),
        }
    }

    pub fn owner_doc(&self) -> DomRoot<Document> {
        self.owner_doc.get().unwrap()
    }

    pub fn set_owner_doc(&self, document: &Document) {
        self.owner_doc.set(Some(document));
    }

    pub fn is_in_html_doc(&self) -> bool {
        self.owner_doc().is_html_document()
    }

    pub fn is_in_doc_with_browsing_context(&self) -> bool {
        self.is_in_doc() && self.owner_doc().browsing_context().is_some()
    }

    pub fn children(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: self.GetFirstChild(),
            next_node: |n| n.GetNextSibling(),
        }
    }

    pub fn rev_children(&self) -> impl Iterator<Item = DomRoot<Node>> {
        SimpleNodeIterator {
            current: self.GetLastChild(),
            next_node: |n| n.GetPreviousSibling(),
        }
    }

    pub fn child_elements(&self) -> impl Iterator<Item = DomRoot<Element>> {
        self.children()
            .filter_map(DomRoot::downcast as fn(_) -> _)
            .peekable()
    }

    pub fn remove_self(&self) {
        if let Some(ref parent) = self.GetParentNode() {
            Node::remove(self, &parent, SuppressObserver::Unsuppressed);
        }
    }

    pub fn unique_id(&self) -> String {
        self.unique_id.borrow().simple().to_string()
    }

    pub fn summarize(&self) -> NodeInfo {
        let USVString(base_uri) = self.BaseURI();
        NodeInfo {
            uniqueId: self.unique_id(),
            baseURI: base_uri,
            parent: self
                .GetParentNode()
                .map_or("".to_owned(), |node| node.unique_id()),
            nodeType: self.NodeType(),
            namespaceURI: String::new(), //FIXME
            nodeName: String::from(self.NodeName()),
            numChildren: self.ChildNodes().Length() as usize,

            //FIXME doctype nodes only
            name: String::new(),
            publicId: String::new(),
            systemId: String::new(),
            attrs: self.downcast().map(Element::summarize).unwrap_or(vec![]),

            isDocumentElement: self
                .owner_doc()
                .GetDocumentElement()
                .map_or(false, |elem| elem.upcast::<Node>() == self),

            shortValue: self.GetNodeValue().map(String::from).unwrap_or_default(), //FIXME: truncate
            incompleteValue: false, //FIXME: reflect truncation
        }
    }

    /// Used by `HTMLTableSectionElement::InsertRow` and `HTMLTableRowElement::InsertCell`
    pub fn insert_cell_or_row<F, G, I>(
        &self,
        index: i32,
        get_items: F,
        new_child: G,
    ) -> Fallible<DomRoot<HTMLElement>>
    where
        F: Fn() -> DomRoot<HTMLCollection>,
        G: Fn() -> DomRoot<I>,
        I: DerivedFrom<Node> + DerivedFrom<HTMLElement> + DomObject,
    {
        if index < -1 {
            return Err(Error::IndexSize);
        }

        let tr = new_child();

        {
            let tr_node = tr.upcast::<Node>();
            if index == -1 {
                self.InsertBefore(tr_node, None)?;
            } else {
                let items = get_items();
                let node = match items
                    .elements_iter()
                    .map(DomRoot::upcast::<Node>)
                    .map(Some)
                    .chain(iter::once(None))
                    .nth(index as usize)
                {
                    None => return Err(Error::IndexSize),
                    Some(node) => node,
                };
                self.InsertBefore(tr_node, node.r())?;
            }
        }

        Ok(DomRoot::upcast::<HTMLElement>(tr))
    }

    /// Used by `HTMLTableSectionElement::DeleteRow` and `HTMLTableRowElement::DeleteCell`
    pub fn delete_cell_or_row<F, G>(
        &self,
        index: i32,
        get_items: F,
        is_delete_type: G,
    ) -> ErrorResult
    where
        F: Fn() -> DomRoot<HTMLCollection>,
        G: Fn(&Element) -> bool,
    {
        let element = match index {
            index if index < -1 => return Err(Error::IndexSize),
            -1 => {
                let last_child = self.upcast::<Node>().GetLastChild();
                match last_child.and_then(|node| {
                    node.inclusively_preceding_siblings()
                        .filter_map(DomRoot::downcast::<Element>)
                        .filter(|elem| is_delete_type(elem))
                        .next()
                }) {
                    Some(element) => element,
                    None => return Ok(()),
                }
            },
            index => match get_items().Item(index as u32) {
                Some(element) => element,
                None => return Err(Error::IndexSize),
            },
        };

        element.upcast::<Node>().remove_self();
        Ok(())
    }

    pub fn get_stylesheet(&self) -> Option<Arc<Stylesheet>> {
        if let Some(node) = self.downcast::<HTMLStyleElement>() {
            node.get_stylesheet()
        } else if let Some(node) = self.downcast::<HTMLLinkElement>() {
            node.get_stylesheet()
        } else if let Some(node) = self.downcast::<HTMLMetaElement>() {
            node.get_stylesheet()
        } else {
            None
        }
    }

    pub fn get_cssom_stylesheet(&self) -> Option<DomRoot<CSSStyleSheet>> {
        if let Some(node) = self.downcast::<HTMLStyleElement>() {
            node.get_cssom_stylesheet()
        } else if let Some(node) = self.downcast::<HTMLLinkElement>() {
            node.get_cssom_stylesheet()
        } else if let Some(node) = self.downcast::<HTMLMetaElement>() {
            node.get_cssom_stylesheet()
        } else {
            None
        }
    }
}

/// Iterate through `nodes` until we find a `Node` that is not in `not_in`
fn first_node_not_in<I>(mut nodes: I, not_in: &[NodeOrString]) -> Option<DomRoot<Node>>
where
    I: Iterator<Item = DomRoot<Node>>,
{
    nodes.find(|node| {
        not_in.iter().all(|n| match *n {
            NodeOrString::Node(ref n) => n != node,
            _ => true,
        })
    })
}

/// If the given untrusted node address represents a valid DOM node in the given runtime,
/// returns it.
#[allow(unsafe_code)]
pub unsafe fn from_untrusted_node_address(
    _runtime: *mut JSRuntime,
    candidate: UntrustedNodeAddress,
) -> DomRoot<Node> {
    // https://github.com/servo/servo/issues/6383
    let candidate: uintptr_t = mem::transmute(candidate.0);
    //        let object: *mut JSObject = jsfriendapi::bindgen::JS_GetAddressableObject(runtime,
    //                                                                                  candidate);
    let object: *mut JSObject = mem::transmute(candidate);
    if object.is_null() {
        panic!("Attempted to create a `Dom<Node>` from an invalid pointer!")
    }
    let boxed_node = conversions::private_from_object(object) as *const Node;
    DomRoot::from_ref(&*boxed_node)
}

#[allow(unsafe_code)]
pub trait LayoutNodeHelpers {
    unsafe fn type_id_for_layout(&self) -> NodeTypeId;

    unsafe fn parent_node_ref(&self) -> Option<LayoutDom<Node>>;
    unsafe fn first_child_ref(&self) -> Option<LayoutDom<Node>>;
    unsafe fn last_child_ref(&self) -> Option<LayoutDom<Node>>;
    unsafe fn prev_sibling_ref(&self) -> Option<LayoutDom<Node>>;
    unsafe fn next_sibling_ref(&self) -> Option<LayoutDom<Node>>;

    unsafe fn owner_doc_for_layout(&self) -> LayoutDom<Document>;

    unsafe fn is_element_for_layout(&self) -> bool;
    unsafe fn get_flag(&self, flag: NodeFlags) -> bool;
    unsafe fn set_flag(&self, flag: NodeFlags, value: bool);

    unsafe fn children_count(&self) -> u32;

    unsafe fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData>;
    unsafe fn init_style_and_layout_data(&self, OpaqueStyleAndLayoutData);
    unsafe fn take_style_and_layout_data(&self) -> OpaqueStyleAndLayoutData;

    fn text_content(&self) -> String;
    fn selection(&self) -> Option<Range<usize>>;
    fn image_url(&self) -> Option<ServoUrl>;
    fn image_density(&self) -> Option<f64>;
    fn image_data(&self) -> Option<(Option<StdArc<Image>>, Option<ImageMetadata>)>;
    fn canvas_data(&self) -> Option<HTMLCanvasData>;
    fn media_data(&self) -> Option<HTMLMediaData>;
    fn svg_data(&self) -> Option<SVGSVGData>;
    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId>;
    fn iframe_pipeline_id(&self) -> Option<PipelineId>;
    fn opaque(&self) -> OpaqueNode;
}

impl LayoutNodeHelpers for LayoutDom<Node> {
    #[inline]
    #[allow(unsafe_code)]
    unsafe fn type_id_for_layout(&self) -> NodeTypeId {
        (*self.unsafe_get()).type_id()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn is_element_for_layout(&self) -> bool {
        (*self.unsafe_get()).is::<Element>()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn parent_node_ref(&self) -> Option<LayoutDom<Node>> {
        (*self.unsafe_get()).parent_node.get_inner_as_layout()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn first_child_ref(&self) -> Option<LayoutDom<Node>> {
        (*self.unsafe_get()).first_child.get_inner_as_layout()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn last_child_ref(&self) -> Option<LayoutDom<Node>> {
        (*self.unsafe_get()).last_child.get_inner_as_layout()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn prev_sibling_ref(&self) -> Option<LayoutDom<Node>> {
        (*self.unsafe_get()).prev_sibling.get_inner_as_layout()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn next_sibling_ref(&self) -> Option<LayoutDom<Node>> {
        (*self.unsafe_get()).next_sibling.get_inner_as_layout()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn owner_doc_for_layout(&self) -> LayoutDom<Document> {
        (*self.unsafe_get())
            .owner_doc
            .get_inner_as_layout()
            .unwrap()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn get_flag(&self, flag: NodeFlags) -> bool {
        (*self.unsafe_get()).flags.get().contains(flag)
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn set_flag(&self, flag: NodeFlags, value: bool) {
        let this = self.unsafe_get();
        let mut flags = (*this).flags.get();

        if value {
            flags.insert(flag);
        } else {
            flags.remove(flag);
        }

        (*this).flags.set(flags);
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn children_count(&self) -> u32 {
        (*self.unsafe_get()).children_count.get()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData> {
        (*self.unsafe_get()).style_and_layout_data.get()
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn init_style_and_layout_data(&self, val: OpaqueStyleAndLayoutData) {
        debug_assert!((*self.unsafe_get()).style_and_layout_data.get().is_none());
        (*self.unsafe_get()).style_and_layout_data.set(Some(val));
    }

    #[inline]
    #[allow(unsafe_code)]
    unsafe fn take_style_and_layout_data(&self) -> OpaqueStyleAndLayoutData {
        let val = (*self.unsafe_get()).style_and_layout_data.get().unwrap();
        (*self.unsafe_get()).style_and_layout_data.set(None);
        val
    }

    #[allow(unsafe_code)]
    fn text_content(&self) -> String {
        if let Some(text) = self.downcast::<Text>() {
            return unsafe { text.upcast().data_for_layout().to_owned() };
        }

        if let Some(input) = self.downcast::<HTMLInputElement>() {
            return unsafe { input.value_for_layout() };
        }

        if let Some(area) = self.downcast::<HTMLTextAreaElement>() {
            return unsafe { area.value_for_layout() };
        }

        panic!("not text!")
    }

    #[allow(unsafe_code)]
    fn selection(&self) -> Option<Range<usize>> {
        if let Some(area) = self.downcast::<HTMLTextAreaElement>() {
            return unsafe { area.selection_for_layout() };
        }

        if let Some(input) = self.downcast::<HTMLInputElement>() {
            return unsafe { input.selection_for_layout() };
        }

        None
    }

    #[allow(unsafe_code)]
    fn image_url(&self) -> Option<ServoUrl> {
        unsafe {
            self.downcast::<HTMLImageElement>()
                .expect("not an image!")
                .image_url()
        }
    }

    #[allow(unsafe_code)]
    fn image_data(&self) -> Option<(Option<StdArc<Image>>, Option<ImageMetadata>)> {
        unsafe {
            self.downcast::<HTMLImageElement>().map(|e| e.image_data())
        }
    }

    #[allow(unsafe_code)]
    fn image_density(&self) -> Option<f64> {
        unsafe {
            self.downcast::<HTMLImageElement>()
                .expect("not an image!")
                .image_density()
        }
    }

    fn canvas_data(&self) -> Option<HTMLCanvasData> {
        self.downcast::<HTMLCanvasElement>()
            .map(|canvas| canvas.data())
    }

    fn media_data(&self) -> Option<HTMLMediaData> {
        self.downcast::<HTMLMediaElement>()
            .map(|media| media.data())
    }

    fn svg_data(&self) -> Option<SVGSVGData> {
        self.downcast::<SVGSVGElement>().map(|svg| svg.data())
    }

    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId> {
        let iframe_element = self
            .downcast::<HTMLIFrameElement>()
            .expect("not an iframe element!");
        iframe_element.browsing_context_id()
    }

    fn iframe_pipeline_id(&self) -> Option<PipelineId> {
        let iframe_element = self
            .downcast::<HTMLIFrameElement>()
            .expect("not an iframe element!");
        iframe_element.pipeline_id()
    }

    #[allow(unsafe_code)]
    fn opaque(&self) -> OpaqueNode {
        unsafe { OpaqueNode(self.get_jsobject() as usize) }
    }
}

//
// Iteration and traversal
//

pub struct FollowingNodeIterator {
    current: Option<DomRoot<Node>>,
    root: DomRoot<Node>,
}

impl FollowingNodeIterator {
    /// Skips iterating the children of the current node
    pub fn next_skipping_children(&mut self) -> Option<DomRoot<Node>> {
        let current = self.current.take()?;
        self.next_skipping_children_impl(current)
    }

    fn next_skipping_children_impl(&mut self, current: DomRoot<Node>) -> Option<DomRoot<Node>> {
        if self.root == current {
            self.current = None;
            return None;
        }

        if let Some(next_sibling) = current.GetNextSibling() {
            self.current = Some(next_sibling);
            return current.GetNextSibling();
        }

        for ancestor in current.inclusive_ancestors() {
            if self.root == ancestor {
                break;
            }
            if let Some(next_sibling) = ancestor.GetNextSibling() {
                self.current = Some(next_sibling);
                return ancestor.GetNextSibling();
            }
        }
        self.current = None;
        None
    }
}

impl Iterator for FollowingNodeIterator {
    type Item = DomRoot<Node>;

    // https://dom.spec.whatwg.org/#concept-tree-following
    fn next(&mut self) -> Option<DomRoot<Node>> {
        let current = self.current.take()?;

        if let Some(first_child) = current.GetFirstChild() {
            self.current = Some(first_child);
            return current.GetFirstChild();
        }

        self.next_skipping_children_impl(current)
    }
}

pub struct PrecedingNodeIterator {
    current: Option<DomRoot<Node>>,
    root: DomRoot<Node>,
}

impl Iterator for PrecedingNodeIterator {
    type Item = DomRoot<Node>;

    // https://dom.spec.whatwg.org/#concept-tree-preceding
    fn next(&mut self) -> Option<DomRoot<Node>> {
        let current = self.current.take()?;

        self.current = if self.root == current {
            None
        } else if let Some(previous_sibling) = current.GetPreviousSibling() {
            if self.root == previous_sibling {
                None
            } else if let Some(last_child) = previous_sibling.descending_last_children().last() {
                Some(last_child)
            } else {
                Some(previous_sibling)
            }
        } else {
            current.GetParentNode()
        };
        self.current.clone()
    }
}

struct SimpleNodeIterator<I>
where
    I: Fn(&Node) -> Option<DomRoot<Node>>,
{
    current: Option<DomRoot<Node>>,
    next_node: I,
}

impl<I> Iterator for SimpleNodeIterator<I>
where
    I: Fn(&Node) -> Option<DomRoot<Node>>,
{
    type Item = DomRoot<Node>;

    fn next(&mut self) -> Option<Self::Item> {
        let current = self.current.take();
        self.current = current.as_ref().and_then(|c| (self.next_node)(c));
        current
    }
}

pub struct TreeIterator {
    current: Option<DomRoot<Node>>,
    depth: usize,
}

impl TreeIterator {
    fn new(root: &Node) -> TreeIterator {
        TreeIterator {
            current: Some(DomRoot::from_ref(root)),
            depth: 0,
        }
    }

    pub fn next_skipping_children(&mut self) -> Option<DomRoot<Node>> {
        let current = self.current.take()?;

        self.next_skipping_children_impl(current)
    }

    fn next_skipping_children_impl(&mut self, current: DomRoot<Node>) -> Option<DomRoot<Node>> {
        for ancestor in current.inclusive_ancestors() {
            if self.depth == 0 {
                break;
            }
            if let Some(next_sibling) = ancestor.GetNextSibling() {
                self.current = Some(next_sibling);
                return Some(current);
            }
            self.depth -= 1;
        }
        debug_assert_eq!(self.depth, 0);
        self.current = None;
        Some(current)
    }
}

impl Iterator for TreeIterator {
    type Item = DomRoot<Node>;

    // https://dom.spec.whatwg.org/#concept-tree-order
    fn next(&mut self) -> Option<DomRoot<Node>> {
        let current = self.current.take()?;
        if let Some(first_child) = current.GetFirstChild() {
            self.current = Some(first_child);
            self.depth += 1;
            return Some(current);
        };

        self.next_skipping_children_impl(current)
    }
}

/// Specifies whether children must be recursively cloned or not.
#[derive(Clone, Copy, MallocSizeOf, PartialEq)]
pub enum CloneChildrenFlag {
    CloneChildren,
    DoNotCloneChildren,
}

fn as_uintptr<T>(t: &T) -> uintptr_t {
    t as *const T as uintptr_t
}

impl Node {
    pub fn reflect_node<N>(
        node: Box<N>,
        document: &Document,
        wrap_fn: unsafe fn(*mut JSContext, &GlobalScope, Box<N>) -> DomRoot<N>,
    ) -> DomRoot<N>
    where
        N: DerivedFrom<Node> + DomObject,
    {
        let window = document.window();
        reflect_dom_object(node, window, wrap_fn)
    }

    pub fn new_inherited(doc: &Document) -> Node {
        Node::new_(NodeFlags::new(), Some(doc))
    }

    #[allow(unrooted_must_root)]
    pub fn new_document_node() -> Node {
        Node::new_(NodeFlags::new() | NodeFlags::IS_IN_DOC, None)
    }

    #[allow(unrooted_must_root)]
    fn new_(flags: NodeFlags, doc: Option<&Document>) -> Node {
        Node {
            eventtarget: EventTarget::new_inherited(),

            parent_node: Default::default(),
            first_child: Default::default(),
            last_child: Default::default(),
            next_sibling: Default::default(),
            prev_sibling: Default::default(),
            owner_doc: MutNullableDom::new(doc),
            child_list: Default::default(),
            children_count: Cell::new(0u32),
            flags: Cell::new(flags),
            inclusive_descendants_version: Cell::new(0),
            ranges: WeakRangeVec::new(),

            style_and_layout_data: Cell::new(None),

            mutation_observers: Default::default(),

            unique_id: UniqueId::new(),
        }
    }

    // https://dom.spec.whatwg.org/#concept-node-adopt
    pub fn adopt(node: &Node, document: &Document) {
        // Step 1.
        let old_doc = node.owner_doc();
        // Step 2.
        node.remove_self();
        // Step 3.
        if &*old_doc != document {
            // Step 3.1.
            for descendant in node.traverse_preorder() {
                descendant.set_owner_doc(document);
            }
            for descendant in node
                .traverse_preorder()
                .filter_map(|d| d.as_custom_element())
            {
                // Step 3.2.
                ScriptThread::enqueue_callback_reaction(
                    &*descendant,
                    CallbackReaction::Adopted(old_doc.clone(), DomRoot::from_ref(document)),
                    None,
                );
            }
            for descendant in node.traverse_preorder() {
                // Step 3.3.
                vtable_for(&descendant).adopting_steps(&old_doc);
            }
        }
    }

    // https://dom.spec.whatwg.org/#concept-node-ensure-pre-insertion-validity
    pub fn ensure_pre_insertion_validity(
        node: &Node,
        parent: &Node,
        child: Option<&Node>,
    ) -> ErrorResult {
        // Step 1.
        match parent.type_id() {
            NodeTypeId::Document(_) | NodeTypeId::DocumentFragment | NodeTypeId::Element(..) => (),
            _ => return Err(Error::HierarchyRequest),
        }

        // Step 2.
        if node.is_inclusive_ancestor_of(parent) {
            return Err(Error::HierarchyRequest);
        }

        // Step 3.
        if let Some(child) = child {
            if !parent.is_parent_of(child) {
                return Err(Error::NotFound);
            }
        }

        // Step 4-5.
        match node.type_id() {
            NodeTypeId::CharacterData(CharacterDataTypeId::Text) => {
                if parent.is::<Document>() {
                    return Err(Error::HierarchyRequest);
                }
            },
            NodeTypeId::DocumentType => {
                if !parent.is::<Document>() {
                    return Err(Error::HierarchyRequest);
                }
            },
            NodeTypeId::DocumentFragment |
            NodeTypeId::Element(_) |
            NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction) |
            NodeTypeId::CharacterData(CharacterDataTypeId::Comment) => (),
            NodeTypeId::Document(_) => return Err(Error::HierarchyRequest),
        }

        // Step 6.
        if parent.is::<Document>() {
            match node.type_id() {
                // Step 6.1
                NodeTypeId::DocumentFragment => {
                    // Step 6.1.1(b)
                    if node.children().any(|c| c.is::<Text>()) {
                        return Err(Error::HierarchyRequest);
                    }
                    match node.child_elements().count() {
                        0 => (),
                        // Step 6.1.2
                        1 => {
                            if !parent.child_elements().next().is_none() {
                                return Err(Error::HierarchyRequest);
                            }
                            if let Some(child) = child {
                                if child
                                    .inclusively_following_siblings()
                                    .any(|child| child.is_doctype())
                                {
                                    return Err(Error::HierarchyRequest);
                                }
                            }
                        },
                        // Step 6.1.1(a)
                        _ => return Err(Error::HierarchyRequest),
                    }
                },
                // Step 6.2
                NodeTypeId::Element(_) => {
                    if !parent.child_elements().next().is_none() {
                        return Err(Error::HierarchyRequest);
                    }
                    if let Some(ref child) = child {
                        if child
                            .inclusively_following_siblings()
                            .any(|child| child.is_doctype())
                        {
                            return Err(Error::HierarchyRequest);
                        }
                    }
                },
                // Step 6.3
                NodeTypeId::DocumentType => {
                    if parent.children().any(|c| c.is_doctype()) {
                        return Err(Error::HierarchyRequest);
                    }
                    match child {
                        Some(child) => {
                            if parent
                                .children()
                                .take_while(|c| &**c != child)
                                .any(|c| c.is::<Element>())
                            {
                                return Err(Error::HierarchyRequest);
                            }
                        },
                        None => {
                            if !parent.child_elements().next().is_none() {
                                return Err(Error::HierarchyRequest);
                            }
                        },
                    }
                },
                NodeTypeId::CharacterData(_) => (),
                NodeTypeId::Document(_) => unreachable!(),
            }
        }
        Ok(())
    }

    // https://dom.spec.whatwg.org/#concept-node-pre-insert
    pub fn pre_insert(node: &Node, parent: &Node, child: Option<&Node>) -> Fallible<DomRoot<Node>> {
        // Step 1.
        Node::ensure_pre_insertion_validity(node, parent, child)?;

        // Steps 2-3.
        let reference_child_root;
        let reference_child = match child {
            Some(child) if child == node => {
                reference_child_root = node.GetNextSibling();
                reference_child_root.r()
            },
            _ => child,
        };

        // Step 4.
        let document = document_from_node(parent);
        Node::adopt(node, &document);

        // Step 5.
        Node::insert(
            node,
            parent,
            reference_child,
            SuppressObserver::Unsuppressed,
        );

        // Step 6.
        Ok(DomRoot::from_ref(node))
    }

    // https://dom.spec.whatwg.org/#concept-node-insert
    fn insert(
        node: &Node,
        parent: &Node,
        child: Option<&Node>,
        suppress_observers: SuppressObserver,
    ) {
        debug_assert!(&*node.owner_doc() == &*parent.owner_doc());
        debug_assert!(child.map_or(true, |child| Some(parent) == child.GetParentNode().r()));

        // Step 1.
        let count = if node.is::<DocumentFragment>() {
            node.children_count()
        } else {
            1
        };
        // Step 2.
        if let Some(child) = child {
            if !parent.ranges.is_empty() {
                let index = child.index();
                // Steps 2.1-2.
                parent.ranges.increase_above(parent, index, count);
            }
        }
        rooted_vec!(let mut new_nodes);
        let new_nodes = if let NodeTypeId::DocumentFragment = node.type_id() {
            // Step 3.
            new_nodes.extend(node.children().map(|kid| Dom::from_ref(&*kid)));
            // Step 4.
            for kid in new_nodes.r() {
                Node::remove(*kid, node, SuppressObserver::Suppressed);
            }
            // Step 5.
            vtable_for(&node).children_changed(&ChildrenMutation::replace_all(new_nodes.r(), &[]));

            let mutation = Mutation::ChildList {
                added: None,
                removed: Some(new_nodes.r()),
                prev: None,
                next: None,
            };
            MutationObserver::queue_a_mutation_record(&node, mutation);

            new_nodes.r()
        } else {
            // Step 3.
            ref_slice(&node)
        };
        // Step 6.
        let previous_sibling = match suppress_observers {
            SuppressObserver::Unsuppressed => match child {
                Some(child) => child.GetPreviousSibling(),
                None => parent.GetLastChild(),
            },
            SuppressObserver::Suppressed => None,
        };
        // Step 7.
        for kid in new_nodes {
            // Step 7.1.
            parent.add_child(*kid, child);
            // Step 7.7.
            for descendant in kid
                .traverse_preorder()
                .filter_map(DomRoot::downcast::<Element>)
            {
                // Step 7.7.2.
                if descendant.is_connected() {
                    if descendant.get_custom_element_definition().is_some() {
                        // Step 7.7.2.1.
                        ScriptThread::enqueue_callback_reaction(
                            &*descendant,
                            CallbackReaction::Connected,
                            None,
                        );
                    } else {
                        // Step 7.7.2.2.
                        try_upgrade_element(&*descendant);
                    }
                }
            }
        }
        if let SuppressObserver::Unsuppressed = suppress_observers {
            vtable_for(&parent).children_changed(&ChildrenMutation::insert(
                previous_sibling.r(),
                new_nodes,
                child,
            ));

            let mutation = Mutation::ChildList {
                added: Some(new_nodes),
                removed: None,
                prev: previous_sibling.r(),
                next: child,
            };
            MutationObserver::queue_a_mutation_record(&parent, mutation);
        }
    }

    // https://dom.spec.whatwg.org/#concept-node-replace-all
    pub fn replace_all(node: Option<&Node>, parent: &Node) {
        // Step 1.
        if let Some(node) = node {
            Node::adopt(node, &*parent.owner_doc());
        }
        // Step 2.
        rooted_vec!(let removed_nodes <- parent.children());
        // Step 3.
        rooted_vec!(let mut added_nodes);
        let added_nodes = if let Some(node) = node.as_ref() {
            if let NodeTypeId::DocumentFragment = node.type_id() {
                added_nodes.extend(node.children().map(|child| Dom::from_ref(&*child)));
                added_nodes.r()
            } else {
                ref_slice(node)
            }
        } else {
            &[] as &[&Node]
        };
        // Step 4.
        for child in removed_nodes.r() {
            Node::remove(*child, parent, SuppressObserver::Suppressed);
        }
        // Step 5.
        if let Some(node) = node {
            Node::insert(node, parent, None, SuppressObserver::Suppressed);
        }
        // Step 6.
        vtable_for(&parent).children_changed(&ChildrenMutation::replace_all(
            removed_nodes.r(),
            added_nodes,
        ));

        if !removed_nodes.is_empty() || !added_nodes.is_empty() {
            let mutation = Mutation::ChildList {
                added: Some(added_nodes),
                removed: Some(removed_nodes.r()),
                prev: None,
                next: None,
            };
            MutationObserver::queue_a_mutation_record(&parent, mutation);
        }
    }

    // https://dom.spec.whatwg.org/#concept-node-pre-remove
    fn pre_remove(child: &Node, parent: &Node) -> Fallible<DomRoot<Node>> {
        // Step 1.
        match child.GetParentNode() {
            Some(ref node) if &**node != parent => return Err(Error::NotFound),
            None => return Err(Error::NotFound),
            _ => (),
        }

        // Step 2.
        Node::remove(child, parent, SuppressObserver::Unsuppressed);

        // Step 3.
        Ok(DomRoot::from_ref(child))
    }

    // https://dom.spec.whatwg.org/#concept-node-remove
    fn remove(node: &Node, parent: &Node, suppress_observers: SuppressObserver) {
        assert!(
            node.GetParentNode()
                .map_or(false, |node_parent| &*node_parent == parent)
        );
        let cached_index = {
            if parent.ranges.is_empty() {
                None
            } else {
                // Step 1.
                let index = node.index();
                // Steps 2-3 are handled in Node::unbind_from_tree.
                // Steps 4-5.
                parent.ranges.decrease_above(parent, index, 1);
                // Parent had ranges, we needed the index, let's keep track of
                // it to avoid computing it for other ranges when calling
                // unbind_from_tree recursively.
                Some(index)
            }
        };
        // Step 6. pre-removing steps for node iterators
        // Step 7.
        let old_previous_sibling = node.GetPreviousSibling();
        // Step 8.
        let old_next_sibling = node.GetNextSibling();
        // Steps 9-10 are handled in unbind_from_tree.
        parent.remove_child(node, cached_index);
        // Step 11. transient registered observers
        // Step 12.
        if let SuppressObserver::Unsuppressed = suppress_observers {
            vtable_for(&parent).children_changed(&ChildrenMutation::replace(
                old_previous_sibling.r(),
                &Some(&node),
                &[],
                old_next_sibling.r(),
            ));

            let removed = [node];
            let mutation = Mutation::ChildList {
                added: None,
                removed: Some(&removed),
                prev: old_previous_sibling.r(),
                next: old_next_sibling.r(),
            };
            MutationObserver::queue_a_mutation_record(&parent, mutation);
        }
    }

    // https://dom.spec.whatwg.org/#concept-node-clone
    pub fn clone(
        node: &Node,
        maybe_doc: Option<&Document>,
        clone_children: CloneChildrenFlag,
    ) -> DomRoot<Node> {
        // Step 1.
        let document = match maybe_doc {
            Some(doc) => DomRoot::from_ref(doc),
            None => node.owner_doc(),
        };

        // Step 2.
        // XXXabinader: clone() for each node as trait?
        let copy: DomRoot<Node> = match node.type_id() {
            NodeTypeId::DocumentType => {
                let doctype = node.downcast::<DocumentType>().unwrap();
                let doctype = DocumentType::new(
                    doctype.name().clone(),
                    Some(doctype.public_id().clone()),
                    Some(doctype.system_id().clone()),
                    &document,
                );
                DomRoot::upcast::<Node>(doctype)
            },
            NodeTypeId::DocumentFragment => {
                let doc_fragment = DocumentFragment::new(&document);
                DomRoot::upcast::<Node>(doc_fragment)
            },
            NodeTypeId::CharacterData(_) => {
                let cdata = node.downcast::<CharacterData>().unwrap();
                cdata.clone_with_data(cdata.Data(), &document)
            },
            NodeTypeId::Document(_) => {
                let document = node.downcast::<Document>().unwrap();
                let is_html_doc = if document.is_html_document() {
                    IsHTMLDocument::HTMLDocument
                } else {
                    IsHTMLDocument::NonHTMLDocument
                };
                let window = document.window();
                let loader = DocumentLoader::new(&*document.loader());
                let document = Document::new(
                    window,
                    HasBrowsingContext::No,
                    Some(document.url()),
                    // https://github.com/whatwg/dom/issues/378
                    document.origin().clone(),
                    is_html_doc,
                    None,
                    None,
                    DocumentActivity::Inactive,
                    DocumentSource::NotFromParser,
                    loader,
                    None,
                    None,
                    Default::default(),
                );
                DomRoot::upcast::<Node>(document)
            },
            NodeTypeId::Element(..) => {
                let element = node.downcast::<Element>().unwrap();
                let name = QualName {
                    prefix: element.prefix().as_ref().map(|p| Prefix::from(&**p)),
                    ns: element.namespace().clone(),
                    local: element.local_name().clone(),
                };
                let element = Element::create(
                    name,
                    element.get_is(),
                    &document,
                    ElementCreator::ScriptCreated,
                    CustomElementCreationMode::Asynchronous,
                );
                DomRoot::upcast::<Node>(element)
            },
        };

        // Step 3.
        let document = match copy.downcast::<Document>() {
            Some(doc) => DomRoot::from_ref(doc),
            None => DomRoot::from_ref(&*document),
        };
        assert!(copy.owner_doc() == document);

        // Step 4 (some data already copied in step 2).
        match node.type_id() {
            NodeTypeId::Document(_) => {
                let node_doc = node.downcast::<Document>().unwrap();
                let copy_doc = copy.downcast::<Document>().unwrap();
                copy_doc.set_encoding(node_doc.encoding());
                copy_doc.set_quirks_mode(node_doc.quirks_mode());
            },
            NodeTypeId::Element(..) => {
                let node_elem = node.downcast::<Element>().unwrap();
                let copy_elem = copy.downcast::<Element>().unwrap();

                for attr in node_elem.attrs().iter() {
                    copy_elem.push_new_attribute(
                        attr.local_name().clone(),
                        attr.value().clone(),
                        attr.name().clone(),
                        attr.namespace().clone(),
                        attr.prefix().cloned(),
                    );
                }
            },
            _ => (),
        }

        // Step 5: cloning steps.
        vtable_for(&node).cloning_steps(&copy, maybe_doc, clone_children);

        // Step 6.
        if clone_children == CloneChildrenFlag::CloneChildren {
            for child in node.children() {
                let child_copy = Node::clone(&child, Some(&document), clone_children);
                let _inserted_node = Node::pre_insert(&child_copy, &copy, None);
            }
        }

        // Step 7.
        copy
    }

    /// <https://html.spec.whatwg.org/multipage/#child-text-content>
    pub fn child_text_content(&self) -> DOMString {
        Node::collect_text_contents(self.children())
    }

    pub fn collect_text_contents<T: Iterator<Item = DomRoot<Node>>>(iterator: T) -> DOMString {
        let mut content = String::new();
        for node in iterator {
            if let Some(ref text) = node.downcast::<Text>() {
                content.push_str(&text.upcast::<CharacterData>().data());
            }
        }
        DOMString::from(content)
    }

    pub fn namespace_to_string(namespace: Namespace) -> Option<DOMString> {
        match namespace {
            ns!() => None,
            // FIXME(ajeffrey): convert directly from Namespace to DOMString
            _ => Some(DOMString::from(&*namespace)),
        }
    }

    // https://dom.spec.whatwg.org/#locate-a-namespace
    pub fn locate_namespace(node: &Node, prefix: Option<DOMString>) -> Namespace {
        match node.type_id() {
            NodeTypeId::Element(_) => node.downcast::<Element>().unwrap().locate_namespace(prefix),
            NodeTypeId::Document(_) => node
                .downcast::<Document>()
                .unwrap()
                .GetDocumentElement()
                .as_ref()
                .map_or(ns!(), |elem| elem.locate_namespace(prefix)),
            NodeTypeId::DocumentType | NodeTypeId::DocumentFragment => ns!(),
            _ => node
                .GetParentElement()
                .as_ref()
                .map_or(ns!(), |elem| elem.locate_namespace(prefix)),
        }
    }
}

impl NodeMethods for Node {
    // https://dom.spec.whatwg.org/#dom-node-nodetype
    fn NodeType(&self) -> u16 {
        match self.type_id() {
            NodeTypeId::CharacterData(CharacterDataTypeId::Text) => NodeConstants::TEXT_NODE,
            NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction) => {
                NodeConstants::PROCESSING_INSTRUCTION_NODE
            },
            NodeTypeId::CharacterData(CharacterDataTypeId::Comment) => NodeConstants::COMMENT_NODE,
            NodeTypeId::Document(_) => NodeConstants::DOCUMENT_NODE,
            NodeTypeId::DocumentType => NodeConstants::DOCUMENT_TYPE_NODE,
            NodeTypeId::DocumentFragment => NodeConstants::DOCUMENT_FRAGMENT_NODE,
            NodeTypeId::Element(_) => NodeConstants::ELEMENT_NODE,
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-nodename
    fn NodeName(&self) -> DOMString {
        match self.type_id() {
            NodeTypeId::Element(..) => self.downcast::<Element>().unwrap().TagName(),
            NodeTypeId::CharacterData(CharacterDataTypeId::Text) => DOMString::from("#text"),
            NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction) => {
                self.downcast::<ProcessingInstruction>().unwrap().Target()
            },
            NodeTypeId::CharacterData(CharacterDataTypeId::Comment) => DOMString::from("#comment"),
            NodeTypeId::DocumentType => self.downcast::<DocumentType>().unwrap().name().clone(),
            NodeTypeId::DocumentFragment => DOMString::from("#document-fragment"),
            NodeTypeId::Document(_) => DOMString::from("#document"),
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-baseuri
    fn BaseURI(&self) -> USVString {
        USVString(String::from(self.owner_doc().base_url().as_str()))
    }

    // https://dom.spec.whatwg.org/#dom-node-ownerdocument
    fn GetOwnerDocument(&self) -> Option<DomRoot<Document>> {
        match self.type_id() {
            NodeTypeId::CharacterData(..) |
            NodeTypeId::Element(..) |
            NodeTypeId::DocumentType |
            NodeTypeId::DocumentFragment => Some(self.owner_doc()),
            NodeTypeId::Document(_) => None,
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-getrootnode
    fn GetRootNode(&self) -> DomRoot<Node> {
        self.inclusive_ancestors().last().unwrap()
    }

    // https://dom.spec.whatwg.org/#dom-node-parentnode
    fn GetParentNode(&self) -> Option<DomRoot<Node>> {
        self.parent_node.get()
    }

    // https://dom.spec.whatwg.org/#dom-node-parentelement
    fn GetParentElement(&self) -> Option<DomRoot<Element>> {
        self.GetParentNode().and_then(DomRoot::downcast)
    }

    // https://dom.spec.whatwg.org/#dom-node-haschildnodes
    fn HasChildNodes(&self) -> bool {
        self.first_child.get().is_some()
    }

    // https://dom.spec.whatwg.org/#dom-node-childnodes
    fn ChildNodes(&self) -> DomRoot<NodeList> {
        self.child_list.or_init(|| {
            let doc = self.owner_doc();
            let window = doc.window();
            NodeList::new_child_list(window, self)
        })
    }

    // https://dom.spec.whatwg.org/#dom-node-firstchild
    fn GetFirstChild(&self) -> Option<DomRoot<Node>> {
        self.first_child.get()
    }

    // https://dom.spec.whatwg.org/#dom-node-lastchild
    fn GetLastChild(&self) -> Option<DomRoot<Node>> {
        self.last_child.get()
    }

    // https://dom.spec.whatwg.org/#dom-node-previoussibling
    fn GetPreviousSibling(&self) -> Option<DomRoot<Node>> {
        self.prev_sibling.get()
    }

    // https://dom.spec.whatwg.org/#dom-node-nextsibling
    fn GetNextSibling(&self) -> Option<DomRoot<Node>> {
        self.next_sibling.get()
    }

    // https://dom.spec.whatwg.org/#dom-node-nodevalue
    fn GetNodeValue(&self) -> Option<DOMString> {
        self.downcast::<CharacterData>().map(CharacterData::Data)
    }

    // https://dom.spec.whatwg.org/#dom-node-nodevalue
    fn SetNodeValue(&self, val: Option<DOMString>) {
        if let Some(character_data) = self.downcast::<CharacterData>() {
            character_data.SetData(val.unwrap_or_default());
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-textcontent
    fn GetTextContent(&self) -> Option<DOMString> {
        match self.type_id() {
            NodeTypeId::DocumentFragment | NodeTypeId::Element(..) => {
                let content = Node::collect_text_contents(self.traverse_preorder());
                Some(content)
            },
            NodeTypeId::CharacterData(..) => {
                let characterdata = self.downcast::<CharacterData>().unwrap();
                Some(characterdata.Data())
            },
            NodeTypeId::DocumentType | NodeTypeId::Document(_) => None,
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-textcontent
    fn SetTextContent(&self, value: Option<DOMString>) {
        let value = value.unwrap_or_default();
        match self.type_id() {
            NodeTypeId::DocumentFragment | NodeTypeId::Element(..) => {
                // Step 1-2.
                let node = if value.is_empty() {
                    None
                } else {
                    Some(DomRoot::upcast(self.owner_doc().CreateTextNode(value)))
                };

                // Step 3.
                Node::replace_all(node.r(), self);
            },
            NodeTypeId::CharacterData(..) => {
                let characterdata = self.downcast::<CharacterData>().unwrap();
                characterdata.SetData(value);
            },
            NodeTypeId::DocumentType | NodeTypeId::Document(_) => {},
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-insertbefore
    fn InsertBefore(&self, node: &Node, child: Option<&Node>) -> Fallible<DomRoot<Node>> {
        Node::pre_insert(node, self, child)
    }

    // https://dom.spec.whatwg.org/#dom-node-appendchild
    fn AppendChild(&self, node: &Node) -> Fallible<DomRoot<Node>> {
        Node::pre_insert(node, self, None)
    }

    // https://dom.spec.whatwg.org/#concept-node-replace
    fn ReplaceChild(&self, node: &Node, child: &Node) -> Fallible<DomRoot<Node>> {
        // Step 1.
        match self.type_id() {
            NodeTypeId::Document(_) | NodeTypeId::DocumentFragment | NodeTypeId::Element(..) => (),
            _ => return Err(Error::HierarchyRequest),
        }

        // Step 2.
        if node.is_inclusive_ancestor_of(self) {
            return Err(Error::HierarchyRequest);
        }

        // Step 3.
        if !self.is_parent_of(child) {
            return Err(Error::NotFound);
        }

        // Step 4-5.
        match node.type_id() {
            NodeTypeId::CharacterData(CharacterDataTypeId::Text) if self.is::<Document>() => {
                return Err(Error::HierarchyRequest)
            },
            NodeTypeId::DocumentType if !self.is::<Document>() => {
                return Err(Error::HierarchyRequest)
            },
            NodeTypeId::Document(_) => return Err(Error::HierarchyRequest),
            _ => (),
        }

        // Step 6.
        if self.is::<Document>() {
            match node.type_id() {
                // Step 6.1
                NodeTypeId::DocumentFragment => {
                    // Step 6.1.1(b)
                    if node.children().any(|c| c.is::<Text>()) {
                        return Err(Error::HierarchyRequest);
                    }
                    match node.child_elements().count() {
                        0 => (),
                        // Step 6.1.2
                        1 => {
                            if self.child_elements().any(|c| c.upcast::<Node>() != child) {
                                return Err(Error::HierarchyRequest);
                            }
                            if child.following_siblings().any(|child| child.is_doctype()) {
                                return Err(Error::HierarchyRequest);
                            }
                        },
                        // Step 6.1.1(a)
                        _ => return Err(Error::HierarchyRequest),
                    }
                },
                // Step 6.2
                NodeTypeId::Element(..) => {
                    if self.child_elements().any(|c| c.upcast::<Node>() != child) {
                        return Err(Error::HierarchyRequest);
                    }
                    if child.following_siblings().any(|child| child.is_doctype()) {
                        return Err(Error::HierarchyRequest);
                    }
                },
                // Step 6.3
                NodeTypeId::DocumentType => {
                    if self.children().any(|c| c.is_doctype() && &*c != child) {
                        return Err(Error::HierarchyRequest);
                    }
                    if self
                        .children()
                        .take_while(|c| &**c != child)
                        .any(|c| c.is::<Element>())
                    {
                        return Err(Error::HierarchyRequest);
                    }
                },
                NodeTypeId::CharacterData(..) => (),
                NodeTypeId::Document(_) => unreachable!(),
            }
        }

        // Step 7-8.
        let child_next_sibling = child.GetNextSibling();
        let node_next_sibling = node.GetNextSibling();
        let reference_child = if child_next_sibling.r() == Some(node) {
            node_next_sibling.r()
        } else {
            child_next_sibling.r()
        };

        // Step 9.
        let previous_sibling = child.GetPreviousSibling();

        // Step 10.
        let document = document_from_node(self);
        Node::adopt(node, &document);

        let removed_child = if node != child {
            // Step 11.
            Node::remove(child, self, SuppressObserver::Suppressed);
            Some(child)
        } else {
            None
        };

        // Step 12.
        rooted_vec!(let mut nodes);
        let nodes = if node.type_id() == NodeTypeId::DocumentFragment {
            nodes.extend(node.children().map(|node| Dom::from_ref(&*node)));
            nodes.r()
        } else {
            ref_slice(&node)
        };

        // Step 13.
        Node::insert(node, self, reference_child, SuppressObserver::Suppressed);

        // Step 14.
        vtable_for(&self).children_changed(&ChildrenMutation::replace(
            previous_sibling.r(),
            &removed_child,
            nodes,
            reference_child,
        ));
        let removed = removed_child.map(|r| [r]);
        let mutation = Mutation::ChildList {
            added: Some(nodes),
            removed: removed.as_ref().map(|r| &r[..]),
            prev: previous_sibling.r(),
            next: reference_child,
        };
        MutationObserver::queue_a_mutation_record(&self, mutation);

        // Step 15.
        Ok(DomRoot::from_ref(child))
    }

    // https://dom.spec.whatwg.org/#dom-node-removechild
    fn RemoveChild(&self, node: &Node) -> Fallible<DomRoot<Node>> {
        Node::pre_remove(node, self)
    }

    // https://dom.spec.whatwg.org/#dom-node-normalize
    fn Normalize(&self) {
        let mut children = self.children().enumerate().peekable();
        while let Some((_, node)) = children.next() {
            if let Some(text) = node.downcast::<Text>() {
                let cdata = text.upcast::<CharacterData>();
                let mut length = cdata.Length();
                if length == 0 {
                    Node::remove(&node, self, SuppressObserver::Unsuppressed);
                    continue;
                }
                while children
                    .peek()
                    .map_or(false, |&(_, ref sibling)| sibling.is::<Text>())
                {
                    let (index, sibling) = children.next().unwrap();
                    sibling
                        .ranges
                        .drain_to_preceding_text_sibling(&sibling, &node, length);
                    self.ranges
                        .move_to_text_child_at(self, index as u32, &node, length as u32);
                    let sibling_cdata = sibling.downcast::<CharacterData>().unwrap();
                    length += sibling_cdata.Length();
                    cdata.append_data(&sibling_cdata.data());
                    Node::remove(&sibling, self, SuppressObserver::Unsuppressed);
                }
            } else {
                node.Normalize();
            }
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-clonenode
    fn CloneNode(&self, deep: bool) -> DomRoot<Node> {
        Node::clone(
            self,
            None,
            if deep {
                CloneChildrenFlag::CloneChildren
            } else {
                CloneChildrenFlag::DoNotCloneChildren
            },
        )
    }

    // https://dom.spec.whatwg.org/#dom-node-isequalnode
    fn IsEqualNode(&self, maybe_node: Option<&Node>) -> bool {
        fn is_equal_doctype(node: &Node, other: &Node) -> bool {
            let doctype = node.downcast::<DocumentType>().unwrap();
            let other_doctype = other.downcast::<DocumentType>().unwrap();
            (*doctype.name() == *other_doctype.name()) &&
                (*doctype.public_id() == *other_doctype.public_id()) &&
                (*doctype.system_id() == *other_doctype.system_id())
        }
        fn is_equal_element(node: &Node, other: &Node) -> bool {
            let element = node.downcast::<Element>().unwrap();
            let other_element = other.downcast::<Element>().unwrap();
            (*element.namespace() == *other_element.namespace()) &&
                (*element.prefix() == *other_element.prefix()) &&
                (*element.local_name() == *other_element.local_name()) &&
                (element.attrs().len() == other_element.attrs().len())
        }
        fn is_equal_processinginstruction(node: &Node, other: &Node) -> bool {
            let pi = node.downcast::<ProcessingInstruction>().unwrap();
            let other_pi = other.downcast::<ProcessingInstruction>().unwrap();
            (*pi.target() == *other_pi.target()) &&
                (*pi.upcast::<CharacterData>().data() ==
                    *other_pi.upcast::<CharacterData>().data())
        }
        fn is_equal_characterdata(node: &Node, other: &Node) -> bool {
            let characterdata = node.downcast::<CharacterData>().unwrap();
            let other_characterdata = other.downcast::<CharacterData>().unwrap();
            *characterdata.data() == *other_characterdata.data()
        }
        fn is_equal_element_attrs(node: &Node, other: &Node) -> bool {
            let element = node.downcast::<Element>().unwrap();
            let other_element = other.downcast::<Element>().unwrap();
            assert!(element.attrs().len() == other_element.attrs().len());
            element.attrs().iter().all(|attr| {
                other_element.attrs().iter().any(|other_attr| {
                    (*attr.namespace() == *other_attr.namespace()) &&
                        (attr.local_name() == other_attr.local_name()) &&
                        (**attr.value() == **other_attr.value())
                })
            })
        }
        fn is_equal_node(this: &Node, node: &Node) -> bool {
            // Step 2.
            if this.NodeType() != node.NodeType() {
                return false;
            }

            match node.type_id() {
                // Step 3.
                NodeTypeId::DocumentType if !is_equal_doctype(this, node) => return false,
                NodeTypeId::Element(..) if !is_equal_element(this, node) => return false,
                NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction)
                    if !is_equal_processinginstruction(this, node) =>
                {
                    return false
                },
                NodeTypeId::CharacterData(CharacterDataTypeId::Text) |
                NodeTypeId::CharacterData(CharacterDataTypeId::Comment)
                    if !is_equal_characterdata(this, node) =>
                {
                    return false
                },
                // Step 4.
                NodeTypeId::Element(..) if !is_equal_element_attrs(this, node) => return false,
                _ => (),
            }

            // Step 5.
            if this.children_count() != node.children_count() {
                return false;
            }

            // Step 6.
            this.children()
                .zip(node.children())
                .all(|(child, other_child)| is_equal_node(&child, &other_child))
        }
        match maybe_node {
            // Step 1.
            None => false,
            // Step 2-6.
            Some(node) => is_equal_node(self, node),
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-issamenode
    fn IsSameNode(&self, other_node: Option<&Node>) -> bool {
        match other_node {
            Some(node) => self == node,
            None => false,
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-comparedocumentposition
    fn CompareDocumentPosition(&self, other: &Node) -> u16 {
        // step 1.
        if self == other {
            return 0;
        }

        // FIXME(emilio): This will eventually need to handle attribute nodes.

        let mut self_and_ancestors = self.inclusive_ancestors().collect::<SmallVec<[_; 20]>>();
        let mut other_and_ancestors = other.inclusive_ancestors().collect::<SmallVec<[_; 20]>>();

        if self_and_ancestors.last() != other_and_ancestors.last() {
            let random = as_uintptr(self_and_ancestors.last().unwrap()) <
                as_uintptr(other_and_ancestors.last().unwrap());
            let random = if random {
                NodeConstants::DOCUMENT_POSITION_FOLLOWING
            } else {
                NodeConstants::DOCUMENT_POSITION_PRECEDING
            };

            // Disconnected.
            return random +
                NodeConstants::DOCUMENT_POSITION_DISCONNECTED +
                NodeConstants::DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC;
        }

        let mut parent = self_and_ancestors.pop().unwrap();
        other_and_ancestors.pop().unwrap();

        let mut current_position = cmp::min(self_and_ancestors.len(), other_and_ancestors.len());

        while current_position > 0 {
            current_position -= 1;
            let child_1 = self_and_ancestors.pop().unwrap();
            let child_2 = other_and_ancestors.pop().unwrap();

            if child_1 != child_2 {
                let is_before = parent.children().position(|c| c == child_1).unwrap() <
                    parent.children().position(|c| c == child_2).unwrap();
                // If I am before, `other` is following, and the other way
                // around.
                return if is_before {
                    NodeConstants::DOCUMENT_POSITION_FOLLOWING
                } else {
                    NodeConstants::DOCUMENT_POSITION_PRECEDING
                };
            }

            parent = child_1;
        }

        // We hit the end of one of the parent chains, so one node needs to be
        // contained in the other.
        //
        // If we're the container, return that `other` is contained by us.
        return if self_and_ancestors.len() < other_and_ancestors.len() {
            NodeConstants::DOCUMENT_POSITION_FOLLOWING +
                NodeConstants::DOCUMENT_POSITION_CONTAINED_BY
        } else {
            NodeConstants::DOCUMENT_POSITION_PRECEDING + NodeConstants::DOCUMENT_POSITION_CONTAINS
        };
    }

    // https://dom.spec.whatwg.org/#dom-node-contains
    fn Contains(&self, maybe_other: Option<&Node>) -> bool {
        match maybe_other {
            None => false,
            Some(other) => self.is_inclusive_ancestor_of(other),
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-lookupprefix
    fn LookupPrefix(&self, namespace: Option<DOMString>) -> Option<DOMString> {
        let namespace = namespace_from_domstring(namespace);

        // Step 1.
        if namespace == ns!() {
            return None;
        }

        // Step 2.
        match self.type_id() {
            NodeTypeId::Element(..) => self.downcast::<Element>().unwrap().lookup_prefix(namespace),
            NodeTypeId::Document(_) => self
                .downcast::<Document>()
                .unwrap()
                .GetDocumentElement()
                .and_then(|element| element.lookup_prefix(namespace)),
            NodeTypeId::DocumentType | NodeTypeId::DocumentFragment => None,
            _ => self
                .GetParentElement()
                .and_then(|element| element.lookup_prefix(namespace)),
        }
    }

    // https://dom.spec.whatwg.org/#dom-node-lookupnamespaceuri
    fn LookupNamespaceURI(&self, prefix: Option<DOMString>) -> Option<DOMString> {
        // Step 1.
        let prefix = match prefix {
            Some(ref p) if p.is_empty() => None,
            pre => pre,
        };

        // Step 2.
        Node::namespace_to_string(Node::locate_namespace(self, prefix))
    }

    // https://dom.spec.whatwg.org/#dom-node-isdefaultnamespace
    fn IsDefaultNamespace(&self, namespace: Option<DOMString>) -> bool {
        // Step 1.
        let namespace = namespace_from_domstring(namespace);
        // Steps 2 and 3.
        Node::locate_namespace(self, None) == namespace
    }
}

pub fn document_from_node<T: DerivedFrom<Node> + DomObject>(derived: &T) -> DomRoot<Document> {
    derived.upcast().owner_doc()
}

pub fn window_from_node<T: DerivedFrom<Node> + DomObject>(derived: &T) -> DomRoot<Window> {
    let document = document_from_node(derived);
    DomRoot::from_ref(document.window())
}

impl VirtualMethods for Node {
    fn super_type(&self) -> Option<&VirtualMethods> {
        Some(self.upcast::<EventTarget>() as &VirtualMethods)
    }

    fn children_changed(&self, mutation: &ChildrenMutation) {
        if let Some(ref s) = self.super_type() {
            s.children_changed(mutation);
        }
        if let Some(list) = self.child_list.get() {
            list.as_children_list().children_changed(mutation);
        }
        self.owner_doc().content_and_heritage_changed(self);
    }

    // This handles the ranges mentioned in steps 2-3 when removing a node.
    // https://dom.spec.whatwg.org/#concept-node-remove
    fn unbind_from_tree(&self, context: &UnbindContext) {
        self.super_type().unwrap().unbind_from_tree(context);
        self.ranges.drain_to_parent(context, self);
    }
}

/// A summary of the changes that happened to a node.
#[derive(Clone, Copy, MallocSizeOf, PartialEq)]
pub enum NodeDamage {
    /// The node's `style` attribute changed.
    NodeStyleDamaged,
    /// Other parts of a node changed; attributes, text content, etc.
    OtherNodeDamage,
}

pub enum ChildrenMutation<'a> {
    Append {
        prev: &'a Node,
        added: &'a [&'a Node],
    },
    Insert {
        prev: &'a Node,
        added: &'a [&'a Node],
        next: &'a Node,
    },
    Prepend {
        added: &'a [&'a Node],
        next: &'a Node,
    },
    Replace {
        prev: Option<&'a Node>,
        removed: &'a Node,
        added: &'a [&'a Node],
        next: Option<&'a Node>,
    },
    ReplaceAll {
        removed: &'a [&'a Node],
        added: &'a [&'a Node],
    },
    /// Mutation for when a Text node's data is modified.
    /// This doesn't change the structure of the list, which is what the other
    /// variants' fields are stored for at the moment, so this can just have no
    /// fields.
    ChangeText,
}

impl<'a> ChildrenMutation<'a> {
    fn insert(
        prev: Option<&'a Node>,
        added: &'a [&'a Node],
        next: Option<&'a Node>,
    ) -> ChildrenMutation<'a> {
        match (prev, next) {
            (None, None) => ChildrenMutation::ReplaceAll {
                removed: &[],
                added: added,
            },
            (Some(prev), None) => ChildrenMutation::Append {
                prev: prev,
                added: added,
            },
            (None, Some(next)) => ChildrenMutation::Prepend {
                added: added,
                next: next,
            },
            (Some(prev), Some(next)) => ChildrenMutation::Insert {
                prev: prev,
                added: added,
                next: next,
            },
        }
    }

    fn replace(
        prev: Option<&'a Node>,
        removed: &'a Option<&'a Node>,
        added: &'a [&'a Node],
        next: Option<&'a Node>,
    ) -> ChildrenMutation<'a> {
        if let Some(ref removed) = *removed {
            if let (None, None) = (prev, next) {
                ChildrenMutation::ReplaceAll {
                    removed: ref_slice(removed),
                    added: added,
                }
            } else {
                ChildrenMutation::Replace {
                    prev: prev,
                    removed: *removed,
                    added: added,
                    next: next,
                }
            }
        } else {
            ChildrenMutation::insert(prev, added, next)
        }
    }

    fn replace_all(removed: &'a [&'a Node], added: &'a [&'a Node]) -> ChildrenMutation<'a> {
        ChildrenMutation::ReplaceAll {
            removed: removed,
            added: added,
        }
    }

    /// Get the child that follows the added or removed children.
    /// Currently only used when this mutation might force us to
    /// restyle later children (see HAS_SLOW_SELECTOR_LATER_SIBLINGS and
    /// Element's implementation of VirtualMethods::children_changed).
    pub fn next_child(&self) -> Option<&Node> {
        match *self {
            ChildrenMutation::Append { .. } => None,
            ChildrenMutation::Insert { next, .. } => Some(next),
            ChildrenMutation::Prepend { next, .. } => Some(next),
            ChildrenMutation::Replace { next, .. } => next,
            ChildrenMutation::ReplaceAll { .. } => None,
            ChildrenMutation::ChangeText => None,
        }
    }

    /// If nodes were added or removed at the start or end of a container, return any
    /// previously-existing child whose ":first-child" or ":last-child" status *may* have changed.
    ///
    /// NOTE: This does not check whether the inserted/removed nodes were elements, so in some
    /// cases it will return a false positive.  This doesn't matter for correctness, because at
    /// worst the returned element will be restyled unnecessarily.
    pub fn modified_edge_element(&self) -> Option<DomRoot<Node>> {
        match *self {
            // Add/remove at start of container: Return the first following element.
            ChildrenMutation::Prepend { next, .. } |
            ChildrenMutation::Replace {
                prev: None,
                next: Some(next),
                ..
            } => next
                .inclusively_following_siblings()
                .filter(|node| node.is::<Element>())
                .next(),
            // Add/remove at end of container: Return the last preceding element.
            ChildrenMutation::Append { prev, .. } |
            ChildrenMutation::Replace {
                prev: Some(prev),
                next: None,
                ..
            } => prev
                .inclusively_preceding_siblings()
                .filter(|node| node.is::<Element>())
                .next(),
            // Insert or replace in the middle:
            ChildrenMutation::Insert { prev, next, .. } |
            ChildrenMutation::Replace {
                prev: Some(prev),
                next: Some(next),
                ..
            } => {
                if prev
                    .inclusively_preceding_siblings()
                    .all(|node| !node.is::<Element>())
                {
                    // Before the first element: Return the first following element.
                    next.inclusively_following_siblings()
                        .filter(|node| node.is::<Element>())
                        .next()
                } else if next
                    .inclusively_following_siblings()
                    .all(|node| !node.is::<Element>())
                {
                    // After the last element: Return the last preceding element.
                    prev.inclusively_preceding_siblings()
                        .filter(|node| node.is::<Element>())
                        .next()
                } else {
                    None
                }
            },

            ChildrenMutation::Replace {
                prev: None,
                next: None,
                ..
            } => unreachable!(),
            ChildrenMutation::ReplaceAll { .. } => None,
            ChildrenMutation::ChangeText => None,
        }
    }
}

/// The context of the unbinding from a tree of a node when one of its
/// inclusive ancestors is removed.
pub struct UnbindContext<'a> {
    /// The index of the inclusive ancestor that was removed.
    index: Cell<Option<u32>>,
    /// The parent of the inclusive ancestor that was removed.
    pub parent: &'a Node,
    /// The previous sibling of the inclusive ancestor that was removed.
    prev_sibling: Option<&'a Node>,
    /// The next sibling of the inclusive ancestor that was removed.
    pub next_sibling: Option<&'a Node>,
    /// Whether the tree is in a document.
    pub tree_in_doc: bool,
}

impl<'a> UnbindContext<'a> {
    /// Create a new `UnbindContext` value.
    fn new(parent: &'a Node,
           prev_sibling: Option<&'a Node>,
           next_sibling: Option<&'a Node>,
           cached_index: Option<u32>) -> Self {
        UnbindContext {
            index: Cell::new(cached_index),
            parent: parent,
            prev_sibling: prev_sibling,
            next_sibling: next_sibling,
            tree_in_doc: parent.is_in_doc(),
        }
    }

    /// The index of the inclusive ancestor that was removed from the tree.
    #[allow(unsafe_code)]
    pub fn index(&self) -> u32 {
        if let Some(index) = self.index.get() {
            return index;
        }
        let index = self.prev_sibling.map_or(0, |sibling| sibling.index() + 1);
        self.index.set(Some(index));
        index
    }
}

/// A node's unique ID, for devtools.
struct UniqueId {
    cell: UnsafeCell<Option<Box<Uuid>>>,
}

unsafe_no_jsmanaged_fields!(UniqueId);

impl MallocSizeOf for UniqueId {
    #[allow(unsafe_code)]
    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        if let &Some(ref uuid) = unsafe { &*self.cell.get() } {
            unsafe { ops.malloc_size_of(&**uuid) }
        } else {
            0
        }
    }
}

impl UniqueId {
    /// Create a new `UniqueId` value. The underlying `Uuid` is lazily created.
    fn new() -> UniqueId {
        UniqueId {
            cell: UnsafeCell::new(None),
        }
    }

    /// The Uuid of that unique ID.
    #[allow(unsafe_code)]
    fn borrow(&self) -> &Uuid {
        unsafe {
            let ptr = self.cell.get();
            if (*ptr).is_none() {
                *ptr = Some(Box::new(Uuid::new_v4()));
            }
            &(&*ptr).as_ref().unwrap()
        }
    }
}

impl Into<LayoutNodeType> for NodeTypeId {
    #[inline(always)]
    fn into(self) -> LayoutNodeType {
        match self {
            NodeTypeId::Element(e) => LayoutNodeType::Element(e.into()),
            NodeTypeId::CharacterData(CharacterDataTypeId::Text) => LayoutNodeType::Text,
            x => unreachable!("Layout should not traverse nodes of type {:?}", x),
        }
    }
}

impl Into<LayoutElementType> for ElementTypeId {
    #[inline(always)]
    fn into(self) -> LayoutElementType {
        match self {
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLBRElement) => {
                LayoutElementType::HTMLBRElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLCanvasElement) => {
                LayoutElementType::HTMLCanvasElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLIFrameElement) => {
                LayoutElementType::HTMLIFrameElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLImageElement) => {
                LayoutElementType::HTMLImageElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLMediaElement(_)) => {
                LayoutElementType::HTMLMediaElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLInputElement) => {
                LayoutElementType::HTMLInputElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLObjectElement) => {
                LayoutElementType::HTMLObjectElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLParagraphElement) => {
                LayoutElementType::HTMLParagraphElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTableCellElement) => {
                LayoutElementType::HTMLTableCellElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTableColElement) => {
                LayoutElementType::HTMLTableColElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTableElement) => {
                LayoutElementType::HTMLTableElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTableRowElement) => {
                LayoutElementType::HTMLTableRowElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTableSectionElement) => {
                LayoutElementType::HTMLTableSectionElement
            },
            ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTextAreaElement) => {
                LayoutElementType::HTMLTextAreaElement
            },
            ElementTypeId::SVGElement(SVGElementTypeId::SVGGraphicsElement(
                SVGGraphicsElementTypeId::SVGSVGElement,
            )) => LayoutElementType::SVGSVGElement,
            _ => LayoutElementType::Element,
        }
    }
}

/// Helper trait to insert an element into vector whose elements
/// are maintained in tree order
pub trait VecPreOrderInsertionHelper<T> {
    fn insert_pre_order(&mut self, elem: &T, tree_root: &Node);
}

impl<T> VecPreOrderInsertionHelper<T> for Vec<Dom<T>>
where
    T: DerivedFrom<Node> + DomObject,
{
    /// This algorithm relies on the following assumptions:
    /// * any elements inserted in this vector share the same tree root
    /// * any time an element is removed from the tree root, it is also removed from this array
    /// * any time an element is moved within the tree, it is removed from this array and re-inserted
    ///
    /// Under these assumptions, an element's tree-order position in this array can be determined by
    /// performing a [preorder traversal](https://dom.spec.whatwg.org/#concept-tree-order) of the tree root's children,
    /// and increasing the destination index in the array every time a node in the array is encountered during
    /// the traversal.
    fn insert_pre_order(&mut self, elem: &T, tree_root: &Node) {
        if self.is_empty() {
            self.push(Dom::from_ref(elem));
            return;
        }

        let elem_node = elem.upcast::<Node>();
        let mut head: usize = 0;
        for node in tree_root.traverse_preorder() {
            let head_node = DomRoot::upcast::<Node>(DomRoot::from_ref(&*self[head]));
            if head_node == node {
                head += 1;
            }
            if elem_node == node.r() || head == self.len() {
                break;
            }
        }
        self.insert(head, Dom::from_ref(elem));
    }
}