/*
 * Copyright (c) 2018-2024, Andreas Kling <andreas@ladybird.org>
 * Copyright (c) 2021-2022, Linus Groh <linusg@serenityos.org>
 * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
 * Copyright (c) 2024, Jelle Raaijmakers <jelle@ladybird.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/HashTable.h>
#include <AK/StringBuilder.h>
#include <LibGC/DeferGC.h>
#include <LibJS/Runtime/FunctionObject.h>
#include <LibRegex/Regex.h>
#include <LibWeb/Bindings/MainThreadVM.h>
#include <LibWeb/Bindings/NodePrototype.h>
#include <LibWeb/CSS/StyleComputer.h>
#include <LibWeb/DOM/Attr.h>
#include <LibWeb/DOM/CDATASection.h>
#include <LibWeb/DOM/Comment.h>
#include <LibWeb/DOM/DocumentType.h>
#include <LibWeb/DOM/Element.h>
#include <LibWeb/DOM/ElementFactory.h>
#include <LibWeb/DOM/Event.h>
#include <LibWeb/DOM/EventDispatcher.h>
#include <LibWeb/DOM/IDLEventListener.h>
#include <LibWeb/DOM/LiveNodeList.h>
#include <LibWeb/DOM/MutationType.h>
#include <LibWeb/DOM/NamedNodeMap.h>
#include <LibWeb/DOM/Node.h>
#include <LibWeb/DOM/NodeIterator.h>
#include <LibWeb/DOM/ProcessingInstruction.h>
#include <LibWeb/DOM/Range.h>
#include <LibWeb/DOM/ShadowRoot.h>
#include <LibWeb/DOM/StaticNodeList.h>
#include <LibWeb/DOM/XMLDocument.h>
#include <LibWeb/HTML/CustomElements/CustomElementReactionNames.h>
#include <LibWeb/HTML/HTMLAnchorElement.h>
#include <LibWeb/HTML/HTMLDocument.h>
#include <LibWeb/HTML/HTMLFieldSetElement.h>
#include <LibWeb/HTML/HTMLImageElement.h>
#include <LibWeb/HTML/HTMLInputElement.h>
#include <LibWeb/HTML/HTMLLegendElement.h>
#include <LibWeb/HTML/HTMLSelectElement.h>
#include <LibWeb/HTML/HTMLSlotElement.h>
#include <LibWeb/HTML/HTMLStyleElement.h>
#include <LibWeb/HTML/HTMLTableElement.h>
#include <LibWeb/HTML/HTMLTextAreaElement.h>
#include <LibWeb/HTML/Navigable.h>
#include <LibWeb/HTML/NavigableContainer.h>
#include <LibWeb/HTML/Parser/HTMLParser.h>
#include <LibWeb/Infra/CharacterTypes.h>
#include <LibWeb/Layout/Node.h>
#include <LibWeb/Layout/TextNode.h>
#include <LibWeb/MathML/MathMLElement.h>
#include <LibWeb/Namespace.h>
#include <LibWeb/Painting/Paintable.h>
#include <LibWeb/Painting/PaintableBox.h>
#include <LibWeb/SVG/SVGElement.h>
#include <LibWeb/SVG/SVGTitleElement.h>
#include <LibWeb/XLink/AttributeNames.h>

namespace Web::DOM {

static UniqueNodeID s_next_unique_id;
static HashMap<UniqueNodeID, Node*> s_node_directory;

static UniqueNodeID allocate_unique_id(Node* node)
{
    auto id = s_next_unique_id;
    ++s_next_unique_id;
    s_node_directory.set(id, node);
    return id;
}

static void deallocate_unique_id(UniqueNodeID node_id)
{
    if (!s_node_directory.remove(node_id))
        VERIFY_NOT_REACHED();
}

Node* Node::from_unique_id(UniqueNodeID unique_id)
{
    return s_node_directory.get(unique_id).value_or(nullptr);
}

Node::Node(JS::Realm& realm, Document& document, NodeType type)
    : EventTarget(realm)
    , m_document(&document)
    , m_type(type)
    , m_unique_id(allocate_unique_id(this))
{
}

Node::Node(Document& document, NodeType type)
    : Node(document.realm(), document, type)
{
}

Node::~Node() = default;

void Node::finalize()
{
    Base::finalize();
    deallocate_unique_id(m_unique_id);
}

void Node::visit_edges(Cell::Visitor& visitor)
{
    Base::visit_edges(visitor);
    TreeNode::visit_edges(visitor);
    visitor.visit(m_document);
    visitor.visit(m_child_nodes);

    visitor.visit(m_layout_node);
    visitor.visit(m_paintable);

    if (m_registered_observer_list) {
        visitor.visit(*m_registered_observer_list);
    }
}

// https://dom.spec.whatwg.org/#dom-node-baseuri
String Node::base_uri() const
{
    // Return this’s node document’s document base URL, serialized.
    return document().base_url().to_string();
}

const HTML::HTMLAnchorElement* Node::enclosing_link_element() const
{
    for (auto* node = this; node; node = node->parent()) {
        if (!is<HTML::HTMLAnchorElement>(*node))
            continue;
        auto const& anchor_element = static_cast<HTML::HTMLAnchorElement const&>(*node);
        if (anchor_element.has_attribute(HTML::AttributeNames::href))
            return &anchor_element;
    }
    return nullptr;
}

const HTML::HTMLElement* Node::enclosing_html_element() const
{
    return first_ancestor_of_type<HTML::HTMLElement>();
}

const HTML::HTMLElement* Node::enclosing_html_element_with_attribute(FlyString const& attribute) const
{
    for (auto* node = this; node; node = node->parent()) {
        if (is<HTML::HTMLElement>(*node) && as<HTML::HTMLElement>(*node).has_attribute(attribute))
            return as<HTML::HTMLElement>(node);
    }
    return nullptr;
}

Optional<String> Node::alternative_text() const
{
    return {};
}

// https://dom.spec.whatwg.org/#concept-descendant-text-content
String Node::descendant_text_content() const
{
    StringBuilder builder;
    for_each_in_subtree_of_type<Text>([&](auto& text_node) {
        builder.append(text_node.data());
        return TraversalDecision::Continue;
    });
    return builder.to_string_without_validation();
}

// https://dom.spec.whatwg.org/#dom-node-textcontent
Optional<String> Node::text_content() const
{
    // The textContent getter steps are to return the following, switching on the interface this implements:

    // If DocumentFragment or Element, return the descendant text content of this.
    if (is<DocumentFragment>(this) || is<Element>(this))
        return descendant_text_content();

    // If CharacterData, return this’s data.
    if (is<CharacterData>(this))
        return static_cast<CharacterData const&>(*this).data();

    // If Attr node, return this's value.
    if (is<Attr>(*this))
        return static_cast<Attr const&>(*this).value();

    // Otherwise, return null
    return {};
}

// https://dom.spec.whatwg.org/#ref-for-dom-node-textcontent%E2%91%A0
void Node::set_text_content(Optional<String> const& maybe_content)
{
    // The textContent setter steps are to, if the given value is null, act as if it was the empty string instead,
    // and then do as described below, switching on the interface this implements:
    auto content = maybe_content.value_or(String {});

    // If DocumentFragment or Element, string replace all with the given value within this.
    if (is<DocumentFragment>(this) || is<Element>(this)) {
        // OPTIMIZATION: Replacing nothing with nothing is a no-op. Avoid all invalidation in this case.
        if (!first_child() && content.is_empty()) {
            return;
        }
        string_replace_all(content);
    }

    // If CharacterData, replace data with node this, offset 0, count this’s length, and data the given value.
    else if (is<CharacterData>(this)) {

        auto* character_data_node = as<CharacterData>(this);
        character_data_node->set_data(content);

        // FIXME: CharacterData::set_data is not spec compliant. Make this match the spec when set_data becomes spec compliant.
        //        Do note that this will make this function able to throw an exception.
    }

    // If Attr, set an existing attribute value with this and the given value.
    if (is<Attr>(*this)) {
        static_cast<Attr&>(*this).set_value(content);
    }

    // Otherwise, do nothing.

    if (is_connected()) {
        invalidate_style(StyleInvalidationReason::NodeSetTextContent);
        set_needs_layout_tree_update(true);
    }

    document().bump_dom_tree_version();
}

// https://dom.spec.whatwg.org/#dom-node-normalize
WebIDL::ExceptionOr<void> Node::normalize()
{
    auto contiguous_exclusive_text_nodes_excluding_self = [](Node& node) {
        // https://dom.spec.whatwg.org/#contiguous-exclusive-text-nodes
        // The contiguous exclusive Text nodes of a node node are node, node’s previous sibling exclusive Text node, if any,
        // and its contiguous exclusive Text nodes, and node’s next sibling exclusive Text node, if any,
        // and its contiguous exclusive Text nodes, avoiding any duplicates.
        // NOTE: The callers of this method require node itself to be excluded.
        Vector<Text*> nodes;

        auto* current_node = node.previous_sibling();
        while (current_node && current_node->is_exclusive_text()) {
            nodes.append(static_cast<Text*>(current_node));
            current_node = current_node->previous_sibling();
        }

        // Reverse the order of the nodes so that they are in tree order.
        nodes.reverse();

        current_node = node.next_sibling();
        while (current_node && current_node->is_exclusive_text()) {
            nodes.append(static_cast<Text*>(current_node));
            current_node = current_node->next_sibling();
        }

        return nodes;
    };

    // The normalize() method steps are to run these steps for each descendant exclusive Text node node of this
    Vector<Text&> descendant_exclusive_text_nodes;
    for_each_in_inclusive_subtree_of_type<Text>([&](Text const& node) {
        if (!node.is_cdata_section())
            descendant_exclusive_text_nodes.append(const_cast<Text&>(node));

        return TraversalDecision::Continue;
    });

    for (auto& node : descendant_exclusive_text_nodes) {
        // 1. Let length be node’s length.
        auto& character_data = static_cast<CharacterData&>(node);
        auto length = character_data.length_in_utf16_code_units();

        // 2. If length is zero, then remove node and continue with the next exclusive Text node, if any.
        if (length == 0) {
            if (node.parent())
                node.remove();
            continue;
        }

        // 3. Let data be the concatenation of the data of node’s contiguous exclusive Text nodes (excluding itself), in tree order.
        StringBuilder data;
        for (auto const& text_node : contiguous_exclusive_text_nodes_excluding_self(node))
            data.append(text_node->data());

        // 4. Replace data with node node, offset length, count 0, and data data.
        TRY(character_data.replace_data(length, 0, MUST(data.to_string())));

        // 5. Let currentNode be node’s next sibling.
        auto* current_node = node.next_sibling();

        // 6. While currentNode is an exclusive Text node:
        while (current_node && current_node->is_exclusive_text()) {
            // 1. For each live range whose start node is currentNode, add length to its start offset and set its start node to node.
            for (auto& range : Range::live_ranges()) {
                if (range->start_container() == current_node)
                    TRY(range->set_start(node, range->start_offset() + length));
            }

            // 2. For each live range whose end node is currentNode, add length to its end offset and set its end node to node.
            for (auto& range : Range::live_ranges()) {
                if (range->end_container() == current_node)
                    TRY(range->set_end(node, range->end_offset() + length));
            }

            // 3. For each live range whose start node is currentNode’s parent and start offset is currentNode’s index, set its start node to node and its start offset to length.
            for (auto& range : Range::live_ranges()) {
                if (range->start_container() == current_node->parent() && range->start_offset() == current_node->index())
                    TRY(range->set_start(node, length));
            }

            // 4. For each live range whose end node is currentNode’s parent and end offset is currentNode’s index, set its end node to node and its end offset to length.
            for (auto& range : Range::live_ranges()) {
                if (range->end_container() == current_node->parent() && range->end_offset() == current_node->index())
                    TRY(range->set_end(node, length));
            }

            // 5. Add currentNode’s length to length.
            length += static_cast<Text&>(*current_node).length();

            // 6. Set currentNode to its next sibling.
            current_node = current_node->next_sibling();
        }

        // 7. Remove node’s contiguous exclusive Text nodes (excluding itself), in tree order.
        for (auto const& text_node : contiguous_exclusive_text_nodes_excluding_self(node))
            text_node->remove();
    }

    return {};
}

// https://dom.spec.whatwg.org/#dom-node-nodevalue
Optional<String> Node::node_value() const
{
    // The nodeValue getter steps are to return the following, switching on the interface this implements:

    // If Attr, return this’s value.
    if (is<Attr>(this)) {
        return as<Attr>(this)->value();
    }

    // If CharacterData, return this’s data.
    if (is<CharacterData>(this)) {
        return as<CharacterData>(this)->data();
    }

    // Otherwise, return null.
    return {};
}

// https://dom.spec.whatwg.org/#ref-for-dom-node-nodevalue%E2%91%A0
void Node::set_node_value(Optional<String> const& maybe_value)
{
    // The nodeValue setter steps are to, if the given value is null, act as if it was the empty string instead,
    // and then do as described below, switching on the interface this implements:
    auto value = maybe_value.value_or(String {});

    // If Attr, set an existing attribute value with this and the given value.
    if (is<Attr>(this)) {
        as<Attr>(this)->set_value(move(value));
    } else if (is<CharacterData>(this)) {
        // If CharacterData, replace data with node this, offset 0, count this’s length, and data the given value.
        as<CharacterData>(this)->set_data(value);
    }

    // Otherwise, do nothing.
}

// https://html.spec.whatwg.org/multipage/document-sequences.html#node-navigable
GC::Ptr<HTML::Navigable> Node::navigable() const
{
    auto& document = const_cast<Document&>(this->document());
    if (auto cached_navigable = document.cached_navigable()) {
        if (cached_navigable->active_document() == &document)
            return cached_navigable;
    }

    // To get the node navigable of a node node, return the navigable whose active document is node's node document,
    // or null if there is no such navigable.
    auto navigable = HTML::Navigable::navigable_with_active_document(document);
    document.set_cached_navigable(navigable);
    return navigable;
}

[[maybe_unused]] static StringView to_string(StyleInvalidationReason reason)
{
#define __ENUMERATE_STYLE_INVALIDATION_REASON(reason) \
    case StyleInvalidationReason::reason:             \
        return #reason##sv;
    switch (reason) {
        ENUMERATE_STYLE_INVALIDATION_REASONS(__ENUMERATE_STYLE_INVALIDATION_REASON)
    default:
        VERIFY_NOT_REACHED();
    }
}

void Node::invalidate_style(StyleInvalidationReason reason)
{
    if (is_character_data())
        return;

    if (document().style_computer().may_have_has_selectors()) {
        if (reason == StyleInvalidationReason::NodeRemove) {
            if (auto* parent = parent_or_shadow_host(); parent) {
                document().schedule_ancestors_style_invalidation_due_to_presence_of_has(*parent);
                parent->for_each_child_of_type<Element>([&](auto& element) {
                    if (element.affected_by_has_pseudo_class_with_relative_selector_that_has_sibling_combinator())
                        element.invalidate_style_if_affected_by_has();
                    return IterationDecision::Continue;
                });
            }
        } else {
            document().schedule_ancestors_style_invalidation_due_to_presence_of_has(*this);
        }
    }

    if (!needs_style_update() && !document().needs_full_style_update()) {
        dbgln_if(STYLE_INVALIDATION_DEBUG, "Invalidate style ({}): {}", to_string(reason), debug_description());
    }

    if (is_document()) {
        auto& document = static_cast<DOM::Document&>(*this);
        document.set_needs_full_style_update(true);
        document.schedule_style_update();
        return;
    }

    // If the document is already marked for a full style update, there's no need to do anything here.
    if (document().needs_full_style_update()) {
        return;
    }

    // If any ancestor is already marked for an entire subtree update, there's no need to do anything here.
    for (auto* ancestor = this->parent_or_shadow_host(); ancestor; ancestor = ancestor->parent_or_shadow_host()) {
        if (ancestor->entire_subtree_needs_style_update())
            return;
    }

    // When invalidating style for a node, we actually invalidate:
    // - the node itself
    // - all of its descendants
    // - all of its preceding siblings and their descendants (only on DOM insert/remove)
    // - all of its subsequent siblings and their descendants
    // FIXME: This is a lot of invalidation and we should implement more sophisticated invalidation to do less work!

    set_entire_subtree_needs_style_update(true);

    if (reason == StyleInvalidationReason::NodeInsertBefore || reason == StyleInvalidationReason::NodeRemove) {
        for (auto* sibling = previous_sibling(); sibling; sibling = sibling->previous_sibling()) {
            if (auto* element = as_if<Element>(sibling); element && element->style_affected_by_structural_changes())
                element->set_entire_subtree_needs_style_update(true);
        }
    }

    for (auto* sibling = next_sibling(); sibling; sibling = sibling->next_sibling()) {
        if (auto* element = as_if<Element>(sibling); element && element->style_affected_by_structural_changes())
            element->set_entire_subtree_needs_style_update(true);
    }

    for (auto* ancestor = parent_or_shadow_host(); ancestor; ancestor = ancestor->parent_or_shadow_host())
        ancestor->m_child_needs_style_update = true;

    document().schedule_style_update();
}

void Node::invalidate_style(StyleInvalidationReason reason, Vector<CSS::InvalidationSet::Property> const& properties, StyleInvalidationOptions options)
{
    if (is_character_data())
        return;

    bool properties_used_in_has_selectors = false;
    for (auto const& property : properties) {
        properties_used_in_has_selectors |= document().style_computer().invalidation_property_used_in_has_selector(property);
    }
    if (properties_used_in_has_selectors) {
        document().schedule_ancestors_style_invalidation_due_to_presence_of_has(*this);
    }

    auto invalidation_set = document().style_computer().invalidation_set_for_properties(properties);
    if (options.invalidate_self)
        invalidation_set.set_needs_invalidate_self();
    if (invalidation_set.is_empty())
        return;

    if (invalidation_set.needs_invalidate_whole_subtree()) {
        invalidate_style(reason);
        return;
    }

    if (invalidation_set.needs_invalidate_self()) {
        set_needs_style_update(true);
    }

    auto invalidate_entire_subtree = [&](Node& subtree_root) {
        subtree_root.for_each_shadow_including_inclusive_descendant([&](Node& node) {
            if (!node.is_element())
                return TraversalDecision::Continue;
            auto& element = static_cast<Element&>(node);
            bool needs_style_recalculation = false;
            if (invalidation_set.needs_invalidate_whole_subtree()) {
                VERIFY_NOT_REACHED();
            }

            if (element.includes_properties_from_invalidation_set(invalidation_set)) {
                needs_style_recalculation = true;
            } else if (options.invalidate_elements_that_use_css_custom_properties && element.style_uses_css_custom_properties()) {
                needs_style_recalculation = true;
            }
            if (needs_style_recalculation)
                element.set_needs_style_update(true);
            return TraversalDecision::Continue;
        });
    };

    invalidate_entire_subtree(*this);

    if (invalidation_set.needs_invalidate_whole_subtree()) {
        for (auto* sibling = next_sibling(); sibling; sibling = sibling->next_sibling()) {
            if (sibling->is_element())
                invalidate_entire_subtree(*sibling);
        }
    }

    document().schedule_style_update();
}

String Node::child_text_content() const
{
    if (!is<ParentNode>(*this))
        return String {};

    StringBuilder builder;
    as<ParentNode>(*this).for_each_child([&](auto& child) {
        if (is<Text>(child)) {
            auto maybe_content = as<Text>(child).text_content();
            if (maybe_content.has_value())
                builder.append(maybe_content.value());
        }
        return IterationDecision::Continue;
    });
    return MUST(builder.to_string());
}

// https://dom.spec.whatwg.org/#concept-tree-root
Node& Node::root()
{
    // The root of an object is itself, if its parent is null, or else it is the root of its parent.
    // The root of a tree is any object participating in that tree whose parent is null.
    Node* root = this;
    while (root->parent())
        root = root->parent();
    return *root;
}

// https://dom.spec.whatwg.org/#concept-shadow-including-root
Node& Node::shadow_including_root()
{
    // The shadow-including root of an object is its root’s host’s shadow-including root,
    // if the object’s root is a shadow root; otherwise its root.
    auto& node_root = root();
    if (is<ShadowRoot>(node_root)) {
        if (auto* host = static_cast<ShadowRoot&>(node_root).host(); host)
            return host->shadow_including_root();
    }
    return node_root;
}

// https://dom.spec.whatwg.org/#connected
bool Node::is_connected() const
{
    // An element is connected if its shadow-including root is a document.
    return shadow_including_root().is_document();
}

// https://html.spec.whatwg.org/multipage/infrastructure.html#browsing-context-connected
bool Node::is_browsing_context_connected() const
{
    // A node is browsing-context connected when it is connected and its shadow-including root's browsing context is non-null.
    return is_connected() && shadow_including_root().document().browsing_context();
}

// https://dom.spec.whatwg.org/#concept-node-ensure-pre-insertion-validity
WebIDL::ExceptionOr<void> Node::ensure_pre_insertion_validity(GC::Ref<Node> node, GC::Ptr<Node> child) const
{
    // 1. If parent is not a Document, DocumentFragment, or Element node, then throw a "HierarchyRequestError" DOMException.
    if (!is<Document>(this) && !is<DocumentFragment>(this) && !is<Element>(this))
        return WebIDL::HierarchyRequestError::create(realm(), "Can only insert into a document, document fragment or element"_string);

    // 2. If node is a host-including inclusive ancestor of parent, then throw a "HierarchyRequestError" DOMException.
    if (node->is_host_including_inclusive_ancestor_of(*this))
        return WebIDL::HierarchyRequestError::create(realm(), "New node is an ancestor of this node"_string);

    // 3. If child is non-null and its parent is not parent, then throw a "NotFoundError" DOMException.
    if (child && child->parent() != this)
        return WebIDL::NotFoundError::create(realm(), "This node is not the parent of the given child"_string);

    // FIXME: All the following "Invalid node type for insertion" messages could be more descriptive.
    // 4. If node is not a DocumentFragment, DocumentType, Element, or CharacterData node, then throw a "HierarchyRequestError" DOMException.
    if (!is<DocumentFragment>(*node) && !is<DocumentType>(*node) && !is<Element>(*node) && !is<Text>(*node) && !is<Comment>(*node) && !is<ProcessingInstruction>(*node) && !is<CDATASection>(*node))
        return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);

    // 5. If either node is a Text node and parent is a document, or node is a doctype and parent is not a document, then throw a "HierarchyRequestError" DOMException.
    if ((is<Text>(*node) && is<Document>(this)) || (is<DocumentType>(*node) && !is<Document>(this)))
        return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);

    // 6. If parent is a document, and any of the statements below, switched on the interface node implements, are true, then throw a "HierarchyRequestError" DOMException.
    if (is<Document>(this)) {
        // DocumentFragment
        if (is<DocumentFragment>(*node)) {
            // If node has more than one element child or has a Text node child.
            // Otherwise, if node has one element child and either parent has an element child, child is a doctype, or child is non-null and a doctype is following child.
            auto node_element_child_count = as<DocumentFragment>(*node).child_element_count();
            if ((node_element_child_count > 1 || node->has_child_of_type<Text>())
                || (node_element_child_count == 1 && (has_child_of_type<Element>() || is<DocumentType>(child.ptr()) || (child && child->has_following_node_of_type_in_tree_order<DocumentType>())))) {
                return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);
            }
        } else if (is<Element>(*node)) {
            // Element
            // If parent has an element child, child is a doctype, or child is non-null and a doctype is following child.
            if (has_child_of_type<Element>() || is<DocumentType>(child.ptr()) || (child && child->has_following_node_of_type_in_tree_order<DocumentType>()))
                return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);
        } else if (is<DocumentType>(*node)) {
            // DocumentType
            // parent has a doctype child, child is non-null and an element is preceding child, or child is null and parent has an element child.
            if (has_child_of_type<DocumentType>() || (child && child->has_preceding_node_of_type_in_tree_order<Element>()) || (!child && has_child_of_type<Element>()))
                return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);
        }
    }

    return {};
}

// https://dom.spec.whatwg.org/#concept-node-insert
void Node::insert_before(GC::Ref<Node> node, GC::Ptr<Node> child, bool suppress_observers)
{
    // 1. Let nodes be node’s children, if node is a DocumentFragment node; otherwise « node ».
    Vector<GC::Root<Node>> nodes;
    if (is<DocumentFragment>(*node))
        nodes = node->children_as_vector();
    else
        nodes.append(GC::make_root(*node));

    // 2. Let count be nodes’s size.
    auto count = nodes.size();

    // 3. If count is 0, then return.
    if (count == 0)
        return;

    // 4. If node is a DocumentFragment node, then:
    if (is<DocumentFragment>(*node)) {
        // 1. Remove its children with the suppress observers flag set.
        node->remove_all_children(true);

        // 2. Queue a tree mutation record for node with « », nodes, null, and null.
        // NOTE: This step intentionally does not pay attention to the suppress observers flag.
        node->queue_tree_mutation_record({}, nodes, nullptr, nullptr);
    }

    // 5. If child is non-null, then:
    if (child) {
        // 1. For each live range whose start node is parent and start offset is greater than child’s index, increase its start offset by count.
        for (auto& range : Range::live_ranges()) {
            if (range->start_container() == this && range->start_offset() > child->index())
                range->increase_start_offset({}, count);
        }

        // 2. For each live range whose end node is parent and end offset is greater than child’s index, increase its end offset by count.
        for (auto& range : Range::live_ranges()) {
            if (range->end_container() == this && range->end_offset() > child->index())
                range->increase_end_offset({}, count);
        }
    }

    // 6. Let previousSibling be child’s previous sibling or parent’s last child if child is null.
    GC::Ptr<Node> previous_sibling;
    if (child)
        previous_sibling = child->previous_sibling();
    else
        previous_sibling = last_child();

    // 7. For each node in nodes, in tree order:
    // FIXME: In tree order
    for (auto& node_to_insert : nodes) {
        // 1. Adopt node into parent’s node document.
        document().adopt_node(*node_to_insert);

        // 2. If child is null, then append node to parent’s children.
        if (!child)
            append_child_impl(*node_to_insert);
        // 3. Otherwise, insert node into parent’s children before child’s index.
        else
            insert_before_impl(*node_to_insert, child);

        // 4. If parent is a shadow host whose shadow root’s slot assignment is "named" and node is a slottable, then
        //    assign a slot for node.
        if (is_element()) {
            auto& element = static_cast<DOM::Element&>(*this);

            auto is_named_shadow_host = element.is_shadow_host()
                && element.shadow_root()->slot_assignment() == Bindings::SlotAssignmentMode::Named;

            if (is_named_shadow_host && node_to_insert->is_slottable())
                assign_a_slot(node_to_insert->as_slottable());
        }

        // 5. If parent’s root is a shadow root, and parent is a slot whose assigned nodes is the empty list, then run
        //    signal a slot change for parent.
        if (root().is_shadow_root() && is<HTML::HTMLSlotElement>(*this)) {
            auto& slot = static_cast<HTML::HTMLSlotElement&>(*this);

            if (slot.assigned_nodes_internal().is_empty())
                signal_a_slot_change(slot);
        }

        // 6. Run assign slottables for a tree with node’s root.
        assign_slottables_for_a_tree(node_to_insert->root());

        node_to_insert->invalidate_style(StyleInvalidationReason::NodeInsertBefore);

        // 7. For each shadow-including inclusive descendant inclusiveDescendant of node, in shadow-including tree order:
        node_to_insert->for_each_shadow_including_inclusive_descendant([&](Node& inclusive_descendant) {
            // 1. Run the insertion steps with inclusiveDescendant.
            inclusive_descendant.inserted();

            // 2. If inclusiveDescendant is connected, then:
            // NOTE: This is not specified here in the spec, but these steps can only be performed on an element.
            if (inclusive_descendant.is_connected() && is<DOM::Element>(inclusive_descendant)) {
                auto& element = static_cast<DOM::Element&>(inclusive_descendant);

                // 1. If inclusiveDescendant is custom, then enqueue a custom element callback reaction with inclusiveDescendant,
                //    callback name "connectedCallback", and an empty argument list.
                if (element.is_custom()) {
                    GC::RootVector<JS::Value> empty_arguments { vm().heap() };
                    element.enqueue_a_custom_element_callback_reaction(HTML::CustomElementReactionNames::connectedCallback, move(empty_arguments));
                }

                // 2. Otherwise, try to upgrade inclusiveDescendant.
                // NOTE: If this successfully upgrades inclusiveDescendant, its connectedCallback will be enqueued automatically during
                //       the upgrade an element algorithm.
                else {
                    element.try_to_upgrade();
                }
            }

            return TraversalDecision::Continue;
        });
    }

    // 8. If suppress observers flag is unset, then queue a tree mutation record for parent with nodes, « », previousSibling, and child.
    if (!suppress_observers) {
        queue_tree_mutation_record(nodes, {}, previous_sibling.ptr(), child.ptr());
    }

    // 9. Run the children changed steps for parent.
    ChildrenChangedMetadata metadata { ChildrenChangedMetadata::Type::Inserted, node };
    children_changed(&metadata);

    // 10. Let staticNodeList be a list of nodes, initially « ».
    // Spec-Note: We collect all nodes before calling the post-connection steps on any one of them, instead of calling
    //            the post-connection steps while we’re traversing the node tree. This is because the post-connection
    //            steps can modify the tree’s structure, making live traversal unsafe, possibly leading to the
    //            post-connection steps being called multiple times on the same node.
    GC::RootVector<GC::Ref<Node>> static_node_list(heap());

    // 11. For each node of nodes, in tree order:
    for (auto& node : nodes) {
        // 1. For each shadow-including inclusive descendant inclusiveDescendant of node, in shadow-including tree
        //    order, append inclusiveDescendant to staticNodeList.
        node->for_each_shadow_including_inclusive_descendant([&static_node_list](Node& inclusive_descendant) {
            static_node_list.append(inclusive_descendant);
            return TraversalDecision::Continue;
        });
    }

    // 12. For each node of staticNodeList, if node is connected, then run the post-connection steps with node.
    for (auto& node : static_node_list) {
        if (node->is_connected())
            node->post_connection();
    }

    if (is_connected()) {
        set_needs_layout_tree_update(true);
    }

    document().bump_dom_tree_version();
}

// https://dom.spec.whatwg.org/#concept-node-pre-insert
WebIDL::ExceptionOr<GC::Ref<Node>> Node::pre_insert(GC::Ref<Node> node, GC::Ptr<Node> child)
{
    // 1. Ensure pre-insertion validity of node into parent before child.
    TRY(ensure_pre_insertion_validity(node, child));

    // 2. Let referenceChild be child.
    auto reference_child = child;

    // 3. If referenceChild is node, then set referenceChild to node’s next sibling.
    if (reference_child == node)
        reference_child = node->next_sibling();

    // 4. Insert node into parent before referenceChild.
    insert_before(node, reference_child);

    // 5. Return node.
    return node;
}

// https://dom.spec.whatwg.org/#dom-node-removechild
WebIDL::ExceptionOr<GC::Ref<Node>> Node::remove_child(GC::Ref<Node> child)
{
    // The removeChild(child) method steps are to return the result of pre-removing child from this.
    return pre_remove(child);
}

// https://dom.spec.whatwg.org/#concept-node-pre-remove
WebIDL::ExceptionOr<GC::Ref<Node>> Node::pre_remove(GC::Ref<Node> child)
{
    // 1. If child’s parent is not parent, then throw a "NotFoundError" DOMException.
    if (child->parent() != this)
        return WebIDL::NotFoundError::create(realm(), "Child does not belong to this node"_string);

    // 2. Remove child.
    child->remove();

    // 3. Return child.
    return child;
}

// https://dom.spec.whatwg.org/#concept-node-append
WebIDL::ExceptionOr<GC::Ref<Node>> Node::append_child(GC::Ref<Node> node)
{
    // To append a node to a parent, pre-insert node into parent before null.
    return pre_insert(node, nullptr);
}

// https://dom.spec.whatwg.org/#concept-node-remove
void Node::remove(bool suppress_observers)
{
    // 1. Let parent be node’s parent
    auto* parent = this->parent();

    // 2. Assert: parent is non-null.
    VERIFY(parent);

    // 3. Let index be node’s index.
    auto index = this->index();

    // 4. For each live range whose start node is an inclusive descendant of node, set its start to (parent, index).
    for (auto& range : Range::live_ranges()) {
        if (range->start_container()->is_inclusive_descendant_of(*this))
            MUST(range->set_start(*parent, index));
    }

    // 5. For each live range whose end node is an inclusive descendant of node, set its end to (parent, index).
    for (auto& range : Range::live_ranges()) {
        if (range->end_container()->is_inclusive_descendant_of(*this))
            MUST(range->set_end(*parent, index));
    }

    // 6. For each live range whose start node is parent and start offset is greater than index, decrease its start offset by 1.
    for (auto& range : Range::live_ranges()) {
        if (range->start_container() == parent && range->start_offset() > index)
            range->decrease_start_offset({}, 1);
    }

    // 7. For each live range whose end node is parent and end offset is greater than index, decrease its end offset by 1.
    for (auto& range : Range::live_ranges()) {
        if (range->end_container() == parent && range->end_offset() > index)
            range->decrease_end_offset({}, 1);
    }

    // 8. For each NodeIterator object iterator whose root’s node document is node’s node document, run the NodeIterator pre-removing steps given node and iterator.
    document().for_each_node_iterator([&](NodeIterator& node_iterator) {
        node_iterator.run_pre_removing_steps(*this);
    });

    // 9. Let oldPreviousSibling be node’s previous sibling.
    GC::Ptr<Node> old_previous_sibling = previous_sibling();

    // 10. Let oldNextSibling be node’s next sibling.
    GC::Ptr<Node> old_next_sibling = next_sibling();

    if (is_connected()) {
        // Since the tree structure is about to change, we need to invalidate both style and layout.
        // In the future, we should find a way to only invalidate the parts that actually need it.
        invalidate_style(StyleInvalidationReason::NodeRemove);

        // NOTE: If we didn't have a layout node before, rebuilding the layout tree isn't gonna give us one
        //       after we've been removed from the DOM.
        if (layout_node())
            parent->set_needs_layout_tree_update(true);
    }

    // 11. Remove node from its parent’s children.
    parent->remove_child_impl(*this);

    // 12. If node is assigned, then run assign slottables for node’s assigned slot.
    if (auto assigned_slot = assigned_slot_for_node(*this))
        assign_slottables(*assigned_slot);

    auto& parent_root = parent->root();

    // 13. If parent’s root is a shadow root, and parent is a slot whose assigned nodes is the empty list, then run
    //     signal a slot change for parent.
    if (parent_root.is_shadow_root() && is<HTML::HTMLSlotElement>(parent)) {
        auto& slot = static_cast<HTML::HTMLSlotElement&>(*parent);

        if (slot.assigned_nodes_internal().is_empty())
            signal_a_slot_change(slot);
    }

    // 14. If node has an inclusive descendant that is a slot, then:
    auto has_descendent_slot = false;

    for_each_in_inclusive_subtree_of_type<HTML::HTMLSlotElement>([&](auto const&) {
        has_descendent_slot = true;
        return TraversalDecision::Break;
    });

    if (has_descendent_slot) {
        // 1. Run assign slottables for a tree with parent’s root.
        assign_slottables_for_a_tree(parent_root);

        // 2. Run assign slottables for a tree with node.
        assign_slottables_for_a_tree(*this);
    }

    // 15. Run the removing steps with node and parent.
    removed_from(parent, parent_root);

    // 16. Let isParentConnected be parent’s connected.
    bool is_parent_connected = parent->is_connected();

    // 17. If node is custom and isParentConnected is true, then enqueue a custom element callback reaction with node,
    //     callback name "disconnectedCallback", and an empty argument list.
    // Spec Note: It is intentional for now that custom elements do not get parent passed.
    //            This might change in the future if there is a need.
    if (is<DOM::Element>(*this)) {
        auto& element = static_cast<DOM::Element&>(*this);

        if (element.is_custom() && is_parent_connected) {
            GC::RootVector<JS::Value> empty_arguments { vm().heap() };
            element.enqueue_a_custom_element_callback_reaction(HTML::CustomElementReactionNames::disconnectedCallback, move(empty_arguments));
        }
    }

    // 18. For each shadow-including descendant descendant of node, in shadow-including tree order, then:
    for_each_shadow_including_descendant([&](Node& descendant) {
        // 1. Run the removing steps with descendant
        descendant.removed_from(nullptr, parent_root);

        // 2. If descendant is custom and isParentConnected is true, then enqueue a custom element callback reaction with descendant,
        //    callback name "disconnectedCallback", and an empty argument list.
        if (is<DOM::Element>(descendant)) {
            auto& element = static_cast<DOM::Element&>(descendant);

            if (element.is_custom() && is_parent_connected) {
                GC::RootVector<JS::Value> empty_arguments { vm().heap() };
                element.enqueue_a_custom_element_callback_reaction(HTML::CustomElementReactionNames::disconnectedCallback, move(empty_arguments));
            }
        }

        return TraversalDecision::Continue;
    });

    // 19. For each inclusive ancestor inclusiveAncestor of parent, and then for each registered of inclusiveAncestor’s registered observer list,
    //     if registered’s options["subtree"] is true, then append a new transient registered observer
    //     whose observer is registered’s observer, options is registered’s options, and source is registered to node’s registered observer list.
    for (auto* inclusive_ancestor = parent; inclusive_ancestor; inclusive_ancestor = inclusive_ancestor->parent()) {
        if (!inclusive_ancestor->m_registered_observer_list)
            continue;
        for (auto& registered : *inclusive_ancestor->m_registered_observer_list) {
            if (registered->options().subtree) {
                auto transient_observer = TransientRegisteredObserver::create(registered->observer(), registered->options(), registered);
                add_registered_observer(move(transient_observer));
            }
        }
    }

    // 20. If suppress observers flag is unset, then queue a tree mutation record for parent with « », « node », oldPreviousSibling, and oldNextSibling.
    if (!suppress_observers) {
        parent->queue_tree_mutation_record({}, { *this }, old_previous_sibling.ptr(), old_next_sibling.ptr());
    }

    // 21. Run the children changed steps for parent.
    parent->children_changed(nullptr);

    document().bump_dom_tree_version();
}

// https://dom.spec.whatwg.org/#concept-node-replace
WebIDL::ExceptionOr<GC::Ref<Node>> Node::replace_child(GC::Ref<Node> node, GC::Ref<Node> child)
{
    // If parent is not a Document, DocumentFragment, or Element node, then throw a "HierarchyRequestError" DOMException.
    if (!is<Document>(this) && !is<DocumentFragment>(this) && !is<Element>(this))
        return WebIDL::HierarchyRequestError::create(realm(), "Can only insert into a document, document fragment or element"_string);

    // 2. If node is a host-including inclusive ancestor of parent, then throw a "HierarchyRequestError" DOMException.
    if (node->is_host_including_inclusive_ancestor_of(*this))
        return WebIDL::HierarchyRequestError::create(realm(), "New node is an ancestor of this node"_string);

    // 3. If child’s parent is not parent, then throw a "NotFoundError" DOMException.
    if (child->parent() != this)
        return WebIDL::NotFoundError::create(realm(), "This node is not the parent of the given child"_string);

    // FIXME: All the following "Invalid node type for insertion" messages could be more descriptive.

    // 4. If node is not a DocumentFragment, DocumentType, Element, or CharacterData node, then throw a "HierarchyRequestError" DOMException.
    if (!is<DocumentFragment>(*node) && !is<DocumentType>(*node) && !is<Element>(*node) && !is<Text>(*node) && !is<Comment>(*node) && !is<ProcessingInstruction>(*node))
        return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);

    // 5. If either node is a Text node and parent is a document, or node is a doctype and parent is not a document, then throw a "HierarchyRequestError" DOMException.
    if ((is<Text>(*node) && is<Document>(this)) || (is<DocumentType>(*node) && !is<Document>(this)))
        return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);

    // If parent is a document, and any of the statements below, switched on the interface node implements, are true, then throw a "HierarchyRequestError" DOMException.
    if (is<Document>(this)) {
        // DocumentFragment
        if (is<DocumentFragment>(*node)) {
            // If node has more than one element child or has a Text node child.
            // Otherwise, if node has one element child and either parent has an element child that is not child or a doctype is following child.
            auto node_element_child_count = as<DocumentFragment>(*node).child_element_count();
            if ((node_element_child_count > 1 || node->has_child_of_type<Text>())
                || (node_element_child_count == 1 && (first_child_of_type<Element>() != child || child->has_following_node_of_type_in_tree_order<DocumentType>()))) {
                return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);
            }
        } else if (is<Element>(*node)) {
            // Element
            // parent has an element child that is not child or a doctype is following child.
            if (first_child_of_type<Element>() != child || child->has_following_node_of_type_in_tree_order<DocumentType>())
                return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);
        } else if (is<DocumentType>(*node)) {
            // DocumentType
            // parent has a doctype child that is not child, or an element is preceding child.
            if (first_child_of_type<DocumentType>() != child || child->has_preceding_node_of_type_in_tree_order<Element>())
                return WebIDL::HierarchyRequestError::create(realm(), "Invalid node type for insertion"_string);
        }
    }

    // 7. Let referenceChild be child’s next sibling.
    GC::Ptr<Node> reference_child = child->next_sibling();

    // 8. If referenceChild is node, then set referenceChild to node’s next sibling.
    if (reference_child == node)
        reference_child = node->next_sibling();

    // 9. Let previousSibling be child’s previous sibling.
    GC::Ptr<Node> previous_sibling = child->previous_sibling();

    // 10. Let removedNodes be the empty set.
    Vector<GC::Root<Node>> removed_nodes;

    // 11. If child’s parent is non-null, then:
    // NOTE: The above can only be false if child is node.
    if (child->parent()) {
        // 1. Set removedNodes to « child ».
        removed_nodes.append(GC::make_root(*child));

        // 2. Remove child with the suppress observers flag set.
        child->remove(true);
    }

    // 12. Let nodes be node’s children if node is a DocumentFragment node; otherwise « node ».
    Vector<GC::Root<Node>> nodes;
    if (is<DocumentFragment>(*node))
        nodes = node->children_as_vector();
    else
        nodes.append(GC::make_root(*node));

    // AD-HOC: Since removing the child may have executed arbitrary code, we have to verify
    //         the sanity of inserting `node` before `reference_child` again, as well as
    //         `child` not being reinserted elsewhere.
    if (!reference_child || (reference_child->parent() == this && !child->parent_node())) {
        // 13. Insert node into parent before referenceChild with the suppress observers flag set.
        insert_before(node, reference_child, true);
    }

    // 14. Queue a tree mutation record for parent with nodes, removedNodes, previousSibling, and referenceChild.
    queue_tree_mutation_record(move(nodes), move(removed_nodes), previous_sibling.ptr(), reference_child.ptr());

    // 15. Return child.
    return child;
}

// https://dom.spec.whatwg.org/#concept-node-clone
WebIDL::ExceptionOr<GC::Ref<Node>> Node::clone_node(Document* document, bool subtree, Node* parent) const
{
    // To clone a node given a node node and an optional document document (default node’s node document),
    // boolean subtree (default false), and node-or-null parent (default null):
    if (!document)
        document = m_document;

    // 1. Assert: node is not a document or node is document.
    VERIFY(!is_document() || this == document);

    // 2. Let copy be the result of cloning a single node given node and document.
    auto copy = TRY(clone_single_node(*document));

    // 3. Run any cloning steps defined for node in other applicable specifications and pass node, copy, and subtree as parameters.
    TRY(cloned(*copy, subtree));

    // 4. If parent is non-null, then append copy to parent.
    if (parent)
        TRY(parent->append_child(copy));

    // 5. If subtree is true, then for each child of node’s children, in tree order:
    //    clone a node given child with document set to document, subtree set to subtree, and parent set to copy.
    if (subtree) {
        for (auto child = first_child(); child; child = child->next_sibling()) {
            TRY(child->clone_node(document, subtree, copy));
        }
    }

    // 6. If node is an element, node is a shadow host, and node’s shadow root’s clonable is true:
    if (is_element()) {
        auto& node_element = as<Element>(*this);
        if (node_element.is_shadow_host() && node_element.shadow_root()->clonable()) {
            // 1. Assert: copy is not a shadow host.
            auto& copy_element = as<Element>(*copy);
            VERIFY(!copy_element.is_shadow_host());

            // 2. Attach a shadow root with copy, node’s shadow root’s mode, true, node’s shadow root’s serializable, node’s shadow root’s delegates focus, and node’s shadow root’s slot assignment.
            TRY(copy_element.attach_a_shadow_root(node_element.shadow_root()->mode(), true, node_element.shadow_root()->serializable(), node_element.shadow_root()->delegates_focus(), node_element.shadow_root()->slot_assignment()));

            // 3. Set copy’s shadow root’s declarative to node’s shadow root’s declarative.
            copy_element.shadow_root()->set_declarative(node_element.shadow_root()->declarative());

            // 4. For each child of node’s shadow root’s children, in tree order:
            //    clone a node given child with document set to document, subtree set to subtree, and parent set to copy’s shadow root.
            for (auto child = node_element.shadow_root()->first_child(); child; child = child->next_sibling()) {
                TRY(child->clone_node(document, subtree, copy_element.shadow_root()));
            }
        }
    }

    // 7. Return copy.
    return GC::Ref { *copy };
}

// https://dom.spec.whatwg.org/#clone-a-single-node
WebIDL::ExceptionOr<GC::Ref<Node>> Node::clone_single_node(Document& document) const
{
    // To clone a single node given a node node and document document:

    // 1. Let copy be null.
    GC::Ptr<Node> copy = nullptr;

    // 2. If node is an element:
    if (is_element()) {
        // 1. Set copy to the result of creating an element, given document, node’s local name, node’s namespace, node’s namespace prefix, and node’s is value.
        auto& element = *as<Element>(this);
        auto element_copy = TRY(DOM::create_element(document, element.local_name(), element.namespace_uri(), element.prefix(), element.is_value()));

        // 2. For each attribute of node’s attribute list:
        Optional<WebIDL::Exception> maybe_exception;
        element.for_each_attribute([&](Attr const& attr) {
            // 1. Let copyAttribute be the result of cloning a single node given attribute and document.
            auto copy_attribute_or_error = attr.clone_single_node(document);
            if (copy_attribute_or_error.is_error()) {
                maybe_exception = copy_attribute_or_error.release_error();
                return;
            }

            auto copy_attribute = copy_attribute_or_error.release_value();

            // 2. Append copyAttribute to copy.
            element_copy->append_attribute(as<Attr>(*copy_attribute));
        });

        if (maybe_exception.has_value())
            return *maybe_exception;

        copy = move(element_copy);
    }

    // 3. Otherwise, set copy to a node that implements the same interfaces as node, and fulfills these additional requirements, switching on the interface node implements:
    else {
        if (is_document()) {
            // -> Document
            auto& document_ = as<Document>(*this);
            auto document_copy = [&] -> GC::Ref<Document> {
                switch (document_.document_type()) {
                case Document::Type::XML:
                    return XMLDocument::create(realm(), document_.url());
                case Document::Type::HTML:
                    return HTML::HTMLDocument::create(realm(), document_.url());
                default:
                    return Document::create(realm(), document_.url());
                }
            }();

            // Set copy’s encoding, content type, URL, origin, type, and mode to those of node.
            document_copy->set_encoding(document_.encoding());
            document_copy->set_content_type(document_.content_type());
            document_copy->set_url(document_.url());
            document_copy->set_origin(document_.origin());
            document_copy->set_document_type(document_.document_type());
            document_copy->set_quirks_mode(document_.mode());
            copy = move(document_copy);
        } else if (is_document_type()) {
            // -> DocumentType
            auto& document_type = as<DocumentType>(*this);
            auto document_type_copy = realm().create<DocumentType>(document);

            // Set copy’s name, public ID, and system ID to those of node.
            document_type_copy->set_name(document_type.name());
            document_type_copy->set_public_id(document_type.public_id());
            document_type_copy->set_system_id(document_type.system_id());
            copy = move(document_type_copy);
        } else if (is_attribute()) {
            // -> Attr
            // Set copy’s namespace, namespace prefix, local name, and value to those of node.
            auto& attr = as<Attr>(*this);
            copy = attr.clone(document);
        } else if (is_text()) {
            // -> Text
            auto& text = as<Text>(*this);

            // Set copy’s data to that of node.
            copy = [&]() -> GC::Ref<Text> {
                switch (type()) {
                case NodeType::TEXT_NODE:
                    return realm().create<Text>(document, text.data());
                case NodeType::CDATA_SECTION_NODE:
                    return realm().create<CDATASection>(document, text.data());
                default:
                    VERIFY_NOT_REACHED();
                }
            }();
        } else if (is_comment()) {
            // -> Comment
            auto& comment = as<Comment>(*this);

            // Set copy’s data to that of node.
            auto comment_copy = realm().create<Comment>(document, comment.data());
            copy = move(comment_copy);
        } else if (is<ProcessingInstruction>(this)) {
            // -> ProcessingInstruction
            auto& processing_instruction = as<ProcessingInstruction>(*this);

            // Set copy’s target and data to those of node.
            auto processing_instruction_copy = realm().create<ProcessingInstruction>(document, processing_instruction.data(), processing_instruction.target());
            copy = move(processing_instruction_copy);
        }
        // -> Otherwise
        //    Do nothing.
        else if (is<DocumentFragment>(this)) {
            copy = realm().create<DocumentFragment>(document);
        } else {
            dbgln("Missing code for cloning a '{}' node. Please add it to Node::clone_single_node()", class_name());
            VERIFY_NOT_REACHED();
        }
    }

    // 4. Assert: copy is a node.
    VERIFY(copy);

    // 5. If node is a document, then set document to copy.
    Document& document_to_use = is_document()
        ? static_cast<Document&>(*copy)
        : document;

    // 6. Set copy’s node document to document.
    copy->set_document(document_to_use);

    // 7. Return copy.
    return GC::Ref { *copy };
}

// https://dom.spec.whatwg.org/#dom-node-clonenode
WebIDL::ExceptionOr<GC::Ref<Node>> Node::clone_node_binding(bool subtree)
{
    // 1. If this is a shadow root, then throw a "NotSupportedError" DOMException.
    if (is<ShadowRoot>(*this))
        return WebIDL::NotSupportedError::create(realm(), "Cannot clone shadow root"_string);

    // 2. Return the result of cloning a node given this with subtree set to subtree.
    return clone_node(nullptr, subtree);
}

void Node::set_document(Badge<Document>, Document& document)
{
    set_document(document);
}

void Node::set_document(Badge<NamedNodeMap>, Document& document)
{
    set_document(document);
}

void Node::set_document(Document& document)
{
    if (m_document.ptr() == &document)
        return;

    m_document = &document;

    if (needs_style_update() || child_needs_style_update()) {
        // NOTE: We unset and reset the "needs style update" flag here.
        //       This ensures that there's a pending style update in the new document
        //       that will eventually assign some style to this node if needed.
        set_needs_style_update(false);
        set_needs_style_update(true);
    }
}

// https://w3c.github.io/editing/docs/execCommand/#editable
bool Node::is_editable() const
{
    // Something is editable if it is a node; it is not an editing host;
    if (is_editing_host())
        return false;

    // it does not have a contenteditable attribute set to the false state;
    if (is<HTML::HTMLElement>(this) && static_cast<HTML::HTMLElement const&>(*this).content_editable_state() == HTML::ContentEditableState::False)
        return false;

    // its parent is an editing host or editable;
    if (!parent() || !parent()->is_editable_or_editing_host())
        return false;

    // https://html.spec.whatwg.org/multipage/interaction.html#inert-subtrees
    // When a node is inert:
    // - If it is editable, the node behaves as if it were non-editable.
    if (is_inert())
        return false;

    // and either it is an HTML element,
    if (is<HTML::HTMLElement>(this))
        return true;

    // or it is an svg or math element,
    if (is<SVG::SVGElement>(this) || is<MathML::MathMLElement>(this))
        return true;

    // or it is not an Element and its parent is an HTML element.
    return !is<Element>(this) && is<HTML::HTMLElement>(parent());
}

// https://html.spec.whatwg.org/multipage/interaction.html#editing-host
bool Node::is_editing_host() const
{
    // NOTE: Both conditions below require this to be an HTML element.
    if (!is<HTML::HTMLElement>(this))
        return false;

    // An editing host is either an HTML element with its contenteditable attribute in the true state or
    // plaintext-only state,
    auto state = static_cast<HTML::HTMLElement const&>(*this).content_editable_state();
    if (state == HTML::ContentEditableState::True || state == HTML::ContentEditableState::PlaintextOnly)
        return true;

    // or a child HTML element of a Document whose design mode enabled is true.
    return is<Document>(parent()) && static_cast<Document const&>(*parent()).design_mode_enabled_state();
}

void Node::set_layout_node(Badge<Layout::Node>, GC::Ref<Layout::Node> layout_node)
{
    m_layout_node = layout_node;
}

void Node::detach_layout_node(Badge<Layout::TreeBuilder>)
{
    m_layout_node = nullptr;
}

EventTarget* Node::get_parent(Event const&)
{
    // A node’s get the parent algorithm, given an event, returns the node’s assigned slot, if node is assigned;
    // otherwise node’s parent.
    if (auto assigned_slot = assigned_slot_for_node(*this))
        return assigned_slot.ptr();

    return parent();
}

void Node::set_needs_layout_tree_update(bool value)
{
    if (m_needs_layout_tree_update == value)
        return;
    m_needs_layout_tree_update = value;

    // NOTE: If this is a shadow root, we need to propagate the layout tree update to the host.
    if (is_shadow_root()) {
        auto& shadow_root = static_cast<ShadowRoot&>(*this);
        if (auto host = shadow_root.host())
            host->set_needs_layout_tree_update(value);
    }

    if (m_needs_layout_tree_update) {
        for (auto* ancestor = parent_or_shadow_host(); ancestor; ancestor = ancestor->parent_or_shadow_host()) {
            if (ancestor->m_child_needs_layout_tree_update)
                break;
            ancestor->m_child_needs_layout_tree_update = true;
        }
        document().set_needs_layout();
    }
}

void Node::set_needs_style_update(bool value)
{
    if (m_needs_style_update == value)
        return;
    m_needs_style_update = value;

    if (m_needs_style_update) {
        for (auto* ancestor = parent_or_shadow_host(); ancestor; ancestor = ancestor->parent_or_shadow_host()) {
            if (ancestor->m_child_needs_style_update)
                break;
            ancestor->m_child_needs_style_update = true;
        }
        document().schedule_style_update();
    }
}

void Node::post_connection()
{
}

void Node::inserted()
{
    set_needs_style_update(true);
}

void Node::removed_from(Node*, Node&)
{
    m_layout_node = nullptr;
    m_paintable = nullptr;
}

ParentNode* Node::parent_or_shadow_host()
{
    if (is<ShadowRoot>(*this))
        return static_cast<ShadowRoot&>(*this).host();
    return as<ParentNode>(parent());
}

Element* Node::parent_or_shadow_host_element()
{
    if (is<ShadowRoot>(*this))
        return static_cast<ShadowRoot&>(*this).host();
    if (!parent())
        return nullptr;
    if (is<Element>(*parent()))
        return static_cast<Element*>(parent());
    if (is<ShadowRoot>(*parent()))
        return static_cast<ShadowRoot&>(*parent()).host();
    return nullptr;
}

Slottable Node::as_slottable()
{
    VERIFY(is_slottable());

    if (is_element())
        return GC::Ref { static_cast<Element&>(*this) };
    return GC::Ref { static_cast<Text&>(*this) };
}

GC::Ref<NodeList> Node::child_nodes()
{
    if (!m_child_nodes) {
        m_child_nodes = LiveNodeList::create(realm(), *this, LiveNodeList::Scope::Children, [](auto&) {
            return true;
        });
    }
    return *m_child_nodes;
}

Vector<GC::Root<Node>> Node::children_as_vector() const
{
    Vector<GC::Root<Node>> nodes;

    for_each_child([&](auto& child) {
        nodes.append(GC::make_root(child));
        return IterationDecision::Continue;
    });

    return nodes;
}

void Node::remove_all_children(bool suppress_observers)
{
    while (GC::Ptr<Node> child = first_child())
        child->remove(suppress_observers);
}

// https://dom.spec.whatwg.org/#dom-node-comparedocumentposition
u16 Node::compare_document_position(GC::Ptr<Node> other)
{
    // 1. If this is other, then return zero.
    if (this == other.ptr())
        return DOCUMENT_POSITION_EQUAL;

    // 2. Let node1 be other and node2 be this.
    Node* node1 = other.ptr();
    Node* node2 = this;

    // 3. Let attr1 and attr2 be null.
    Attr* attr1 = nullptr;
    Attr* attr2 = nullptr;

    // 4. If node1 is an attribute, then set attr1 to node1 and node1 to attr1’s element.
    if (is<Attr>(node1)) {
        attr1 = as<Attr>(node1);
        node1 = attr1->owner_element();
    }

    // 5. If node2 is an attribute, then:
    if (is<Attr>(node2)) {
        // 1. Set attr2 to node2 and node2 to attr2’s element.
        attr2 = as<Attr>(node2);
        node2 = attr2->owner_element();

        // 2. If attr1 and node1 are non-null, and node2 is node1, then:
        if (attr1 && node1 && node2 == node1) {
            // FIXME: 1. For each attr of node2’s attribute list:
            //     1. If attr equals attr1, then return the result of adding DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC and DOCUMENT_POSITION_PRECEDING.
            //     2. If attr equals attr2, then return the result of adding DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC and DOCUMENT_POSITION_FOLLOWING.
        }
    }

    // 6. If node1 or node2 is null, or node1’s root is not node2’s root, then return the result of adding
    // DOCUMENT_POSITION_DISCONNECTED, DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC, and either DOCUMENT_POSITION_PRECEDING or DOCUMENT_POSITION_FOLLOWING, with the constraint that this is to be consistent, together.
    if ((node1 == nullptr || node2 == nullptr) || (&node1->root() != &node2->root()))
        return DOCUMENT_POSITION_DISCONNECTED | DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC | (node1 > node2 ? DOCUMENT_POSITION_PRECEDING : DOCUMENT_POSITION_FOLLOWING);

    Vector<Node*> node1_ancestors;
    for (auto* node = node1; node; node = node->parent())
        node1_ancestors.append(node);

    Vector<Node*> node2_ancestors;
    for (auto* node = node2; node; node = node->parent())
        node2_ancestors.append(node);

    auto it_node1_ancestors = node1_ancestors.rbegin();
    auto it_node2_ancestors = node2_ancestors.rbegin();
    // Walk ancestor chains of both nodes starting from root
    while (it_node1_ancestors != node1_ancestors.rend() && it_node2_ancestors != node2_ancestors.rend()) {
        auto* ancestor1 = *it_node1_ancestors;
        auto* ancestor2 = *it_node2_ancestors;

        // If ancestors of nodes at the same level in the tree are different then preceding node is the one with lower sibling position
        if (ancestor1 != ancestor2) {
            auto* node = ancestor1;
            while (node) {
                if (node == ancestor2)
                    return DOCUMENT_POSITION_PRECEDING;
                node = node->next_sibling();
            }
            return DOCUMENT_POSITION_FOLLOWING;
        }

        it_node1_ancestors++;
        it_node2_ancestors++;
    }

    // NOTE: If nodes in ancestors chains are the same but one chain is longer, then one node is ancestor of another.
    //       The node with shorter ancestors chain is the ancestor.
    //       The node with longer ancestors chain is the descendant.

    // 7. If node1 is an ancestor of node2 and attr1 is null, or node1 is node2 and attr2 is non-null, then return the result of adding DOCUMENT_POSITION_CONTAINS to DOCUMENT_POSITION_PRECEDING.
    if ((node1_ancestors.size() < node2_ancestors.size() && !attr1) || (node1 == node2 && attr2))
        return DOCUMENT_POSITION_CONTAINS | DOCUMENT_POSITION_PRECEDING;

    // 8. If node1 is a descendant of node2 and attr2 is null, or node1 is node2 and attr1 is non-null, then return the result of adding DOCUMENT_POSITION_CONTAINED_BY to DOCUMENT_POSITION_FOLLOWING.
    if ((node1_ancestors.size() > node2_ancestors.size() && !attr2) || (node1 == node2 && attr1))
        return DOCUMENT_POSITION_CONTAINED_BY | DOCUMENT_POSITION_FOLLOWING;

    // 9. If node1 is preceding node2, then return DOCUMENT_POSITION_PRECEDING.
    if (node1_ancestors.size() < node2_ancestors.size())
        return DOCUMENT_POSITION_PRECEDING;

    // 10. Return DOCUMENT_POSITION_FOLLOWING.
    return DOCUMENT_POSITION_FOLLOWING;
}

// https://dom.spec.whatwg.org/#concept-tree-host-including-inclusive-ancestor
bool Node::is_host_including_inclusive_ancestor_of(Node const& other) const
{
    // An object A is a host-including inclusive ancestor of an object B,
    // if either A is an inclusive ancestor of B,
    if (is_inclusive_ancestor_of(other))
        return true;

    // or if B’s root has a non-null host and A is a host-including inclusive ancestor of B’s root’s host
    if (is<DocumentFragment>(other.root())
        && static_cast<DocumentFragment const&>(other.root()).host()
        && is_inclusive_ancestor_of(*static_cast<DocumentFragment const&>(other.root()).host())) {
        return true;
    }
    return false;
}

// https://dom.spec.whatwg.org/#dom-node-ownerdocument
GC::Ptr<Document> Node::owner_document() const
{
    // The ownerDocument getter steps are to return null, if this is a document; otherwise this’s node document.
    if (is_document())
        return nullptr;
    return m_document;
}

// This function tells us whether a node is interesting enough to show up
// in the DOM inspector. This hides two things:
// - Non-rendered whitespace
// - Rendered whitespace between block-level elements
bool Node::is_uninteresting_whitespace_node() const
{
    if (!is<Text>(*this))
        return false;
    if (!static_cast<Text const&>(*this).data().bytes_as_string_view().is_whitespace())
        return false;
    if (!layout_node())
        return true;
    if (auto parent = layout_node()->parent(); parent && parent->is_anonymous())
        return true;
    return false;
}

void Node::serialize_tree_as_json(JsonObjectSerializer<StringBuilder>& object) const
{
    MUST(object.add("name"sv, node_name()));
    MUST(object.add("id"sv, unique_id().value()));
    if (is_document()) {
        MUST(object.add("type"sv, "document"));
    } else if (is_element()) {
        MUST(object.add("type"sv, "element"));

        auto const* element = static_cast<DOM::Element const*>(this);
        if (element->namespace_uri().has_value())
            MUST(object.add("namespace"sv, element->namespace_uri().value()));

        if (element->has_attributes()) {
            auto attributes = MUST(object.add_object("attributes"sv));
            element->for_each_attribute([&attributes](auto& name, auto& value) {
                MUST(attributes.add(name, value));
            });
            MUST(attributes.finish());
        }

        if (element->is_navigable_container()) {
            auto const* container = static_cast<HTML::NavigableContainer const*>(element);
            if (auto const* content_document = container->content_document()) {
                auto children = MUST(object.add_array("children"sv));
                JsonObjectSerializer<StringBuilder> content_document_object = MUST(children.add_object());
                content_document->serialize_tree_as_json(content_document_object);
                MUST(content_document_object.finish());
                MUST(children.finish());
            }
        }

        if (paintable_box()) {
            if (paintable_box()->could_be_scrolled_by_wheel_event()) {
                MUST(object.add("scrollable"sv, true));
            }
            if (!paintable_box()->is_visible()) {
                MUST(object.add("invisible"sv, true));
            }
            if (paintable_box()->has_stacking_context()) {
                MUST(object.add("stackingContext"sv, true));
            }
        }
    } else if (is_text()) {
        MUST(object.add("type"sv, "text"));

        auto text_node = static_cast<DOM::Text const*>(this);
        MUST(object.add("text"sv, text_node->data()));
    } else if (is_comment()) {
        MUST(object.add("type"sv, "comment"sv));
        MUST(object.add("data"sv, static_cast<DOM::Comment const&>(*this).data()));
    } else if (is_shadow_root()) {
        MUST(object.add("type"sv, "shadow-root"));
        MUST(object.add("mode"sv, static_cast<DOM::ShadowRoot const&>(*this).mode() == Bindings::ShadowRootMode::Open ? "open"sv : "closed"sv));
    }

    MUST((object.add("visible"sv, !!layout_node())));

    auto const* element = is_element() ? static_cast<DOM::Element const*>(this) : nullptr;

    if (has_child_nodes()
        || (element && (element->is_shadow_host() || element->has_pseudo_elements()))) {
        auto children = MUST(object.add_array("children"sv));
        auto add_child = [&children](DOM::Node const& child) {
            if (child.is_uninteresting_whitespace_node())
                return IterationDecision::Continue;
            JsonObjectSerializer<StringBuilder> child_object = MUST(children.add_object());
            child.serialize_tree_as_json(child_object);
            MUST(child_object.finish());
            return IterationDecision::Continue;
        };
        for_each_child(add_child);

        if (element) {
            // Pseudo-elements don't have DOM nodes,so we have to add them separately.
            element->serialize_pseudo_elements_as_json(children);

            if (element->is_shadow_host())
                add_child(*element->shadow_root());
        }

        MUST(children.finish());
    }
}

// https://html.spec.whatwg.org/multipage/webappapis.html#concept-n-script
// https://whatpr.org/html/9893/webappapis.html#concept-n-script
bool Node::is_scripting_enabled() const
{
    // Scripting is enabled for a node node if node's node document's browsing context is non-null, and scripting is enabled for node's relevant realm.
    return document().browsing_context() && HTML::is_scripting_enabled(HTML::relevant_realm(*this));
}

// https://html.spec.whatwg.org/multipage/webappapis.html#concept-n-noscript
// https://whatpr.org/html/9893/webappapis.html#concept-n-script
bool Node::is_scripting_disabled() const
{
    // Scripting is disabled for a node when scripting is not enabled, i.e., when its node document's browsing context is null or when scripting is disabled for its relevant realm.
    return !is_scripting_enabled();
}

// https://dom.spec.whatwg.org/#dom-node-contains
bool Node::contains(GC::Ptr<Node> other) const
{
    // The contains(other) method steps are to return true if other is an inclusive descendant of this; otherwise false (including when other is null).
    return other && other->is_inclusive_descendant_of(*this);
}

// https://dom.spec.whatwg.org/#concept-shadow-including-descendant
bool Node::is_shadow_including_descendant_of(Node const& other) const
{
    // An object A is a shadow-including descendant of an object B,
    // if A is a descendant of B,
    if (is_descendant_of(other))
        return true;

    // or A’s root is a shadow root
    if (!is<ShadowRoot>(root()))
        return false;

    // and A’s root’s host is a shadow-including inclusive descendant of B.
    auto& shadow_root = as<ShadowRoot>(root());
    return shadow_root.host() && shadow_root.host()->is_shadow_including_inclusive_descendant_of(other);
}

// https://dom.spec.whatwg.org/#concept-shadow-including-inclusive-descendant
bool Node::is_shadow_including_inclusive_descendant_of(Node const& other) const
{
    // A shadow-including inclusive descendant is an object or one of its shadow-including descendants.
    return &other == this || is_shadow_including_descendant_of(other);
}

// https://dom.spec.whatwg.org/#concept-shadow-including-ancestor
bool Node::is_shadow_including_ancestor_of(Node const& other) const
{
    // An object A is a shadow-including ancestor of an object B, if and only if B is a shadow-including descendant of A.
    return other.is_shadow_including_descendant_of(*this);
}

// https://dom.spec.whatwg.org/#concept-shadow-including-inclusive-ancestor
bool Node::is_shadow_including_inclusive_ancestor_of(Node const& other) const
{
    // A shadow-including inclusive ancestor is an object or one of its shadow-including ancestors.
    return other.is_shadow_including_inclusive_descendant_of(*this);
}

// https://dom.spec.whatwg.org/#concept-node-replace-all
void Node::replace_all(GC::Ptr<Node> node)
{
    // 1. Let removedNodes be parent’s children.
    auto removed_nodes = children_as_vector();

    // 2. Let addedNodes be the empty set.
    Vector<GC::Root<Node>> added_nodes;

    // 3. If node is a DocumentFragment node, then set addedNodes to node’s children.
    if (node && is<DocumentFragment>(*node)) {
        added_nodes = node->children_as_vector();
    }
    // 4. Otherwise, if node is non-null, set addedNodes to « node ».
    else if (node) {
        added_nodes.append(GC::make_root(*node));
    }

    // 5. Remove all parent’s children, in tree order, with the suppress observers flag set.
    remove_all_children(true);

    // 6. If node is non-null, then insert node into parent before null with the suppress observers flag set.
    if (node)
        insert_before(*node, nullptr, true);

    // 7. If either addedNodes or removedNodes is not empty, then queue a tree mutation record for parent with addedNodes, removedNodes, null, and null.
    if (!added_nodes.is_empty() || !removed_nodes.is_empty()) {
        queue_tree_mutation_record(move(added_nodes), move(removed_nodes), nullptr, nullptr);
    }
}

// https://dom.spec.whatwg.org/#string-replace-all
void Node::string_replace_all(String const& string)
{
    // 1. Let node be null.
    GC::Ptr<Node> node;

    // 2. If string is not the empty string, then set node to a new Text node whose data is string and node document is parent’s node document.
    if (!string.is_empty())
        node = realm().create<Text>(document(), string);

    // 3. Replace all with node within parent.
    replace_all(node);
}

// https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#fragment-serializing-algorithm-steps
WebIDL::ExceptionOr<String> Node::serialize_fragment(DOMParsing::RequireWellFormed require_well_formed, FragmentSerializationMode fragment_serialization_mode) const
{
    // 1. Let context document be the value of node's node document.
    auto const& context_document = document();

    // 2. If context document is an HTML document, return the result of HTML fragment serialization algorithm with node, false, and « ».
    if (context_document.is_html_document())
        return HTML::HTMLParser::serialize_html_fragment(*this, HTML::HTMLParser::SerializableShadowRoots::No, {}, fragment_serialization_mode);

    // 3. Return the XML serialization of node given require well-formed.
    // AD-HOC: XML serialization algorithm returns the "outer" XML serialization of the node.
    //         For inner, concatenate the serialization of all children.
    if (fragment_serialization_mode == FragmentSerializationMode::Inner) {
        StringBuilder markup;
        for (auto* child = first_child(); child; child = child->next_sibling()) {
            auto child_markup = TRY(DOMParsing::serialize_node_to_xml_string(*child, require_well_formed));
            markup.append(child_markup.bytes_as_string_view());
        }
        return MUST(markup.to_string());
    }
    return DOMParsing::serialize_node_to_xml_string(*this, require_well_formed);
}

// https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#unsafely-set-html
WebIDL::ExceptionOr<void> Node::unsafely_set_html(Element& context_element, StringView html)
{
    // 1. Let newChildren be the result of the HTML fragment parsing algorithm given contextElement, html, and true.
    auto new_children = HTML::HTMLParser::parse_html_fragment(context_element, html, HTML::HTMLParser::AllowDeclarativeShadowRoots::Yes);

    // 2. Let fragment be a new DocumentFragment whose node document is contextElement’s node document.
    auto fragment = realm().create<DocumentFragment>(context_element.document());

    // 3. For each node in newChildren, append node to fragment.
    for (auto& child : new_children)
        // I don't know if this can throw here, but let's be safe.
        (void)TRY(fragment->append_child(*child));

    // 4. Replace all with fragment within contextElement.
    replace_all(fragment);

    return {};
}

// https://dom.spec.whatwg.org/#dom-node-issamenode
bool Node::is_same_node(Node const* other_node) const
{
    // The isSameNode(otherNode) method steps are to return true if otherNode is this; otherwise false.
    return this == other_node;
}

// https://dom.spec.whatwg.org/#dom-node-isequalnode
bool Node::is_equal_node(Node const* other_node) const
{
    // The isEqualNode(otherNode) method steps are to return true if otherNode is non-null and this equals otherNode; otherwise false.
    if (!other_node)
        return false;

    // Fast path for testing a node against itself.
    if (this == other_node)
        return true;

    // A node A equals a node B if all of the following conditions are true:

    // A and B implement the same interfaces.
    if (!node_name().equals_ignoring_ascii_case(other_node->node_name()))
        return false;

    // The following are equal, switching on the interface A implements:
    switch (node_type()) {
    case (u16)NodeType::DOCUMENT_TYPE_NODE: {
        // Its name, public ID, and system ID.
        auto& this_doctype = as<DocumentType>(*this);
        auto& other_doctype = as<DocumentType>(*other_node);
        if (this_doctype.name() != other_doctype.name()
            || this_doctype.public_id() != other_doctype.public_id()
            || this_doctype.system_id() != other_doctype.system_id())
            return false;
        break;
    }
    case (u16)NodeType::ELEMENT_NODE: {
        // Its namespace, namespace prefix, local name, and its attribute list’s size.
        auto& this_element = as<Element>(*this);
        auto& other_element = as<Element>(*other_node);
        if (this_element.namespace_uri() != other_element.namespace_uri()
            || this_element.prefix() != other_element.prefix()
            || this_element.local_name() != other_element.local_name()
            || this_element.attribute_list_size() != other_element.attribute_list_size())
            return false;
        // If A is an element, each attribute in its attribute list has an attribute that equals an attribute in B’s attribute list.
        bool has_same_attributes = true;
        this_element.for_each_attribute([&](auto const& attribute) {
            if (other_element.get_attribute_ns(attribute.namespace_uri(), attribute.local_name()) != attribute.value())
                has_same_attributes = false;
        });
        if (!has_same_attributes)
            return false;
        break;
    }
    case (u16)NodeType::COMMENT_NODE:
    case (u16)NodeType::TEXT_NODE: {
        // Its data.
        auto& this_cdata = as<CharacterData>(*this);
        auto& other_cdata = as<CharacterData>(*other_node);
        if (this_cdata.data() != other_cdata.data())
            return false;
        break;
    }
    case (u16)NodeType::ATTRIBUTE_NODE: {
        // Its namespace, local name, and value.
        auto& this_attr = as<Attr>(*this);
        auto& other_attr = as<Attr>(*other_node);
        if (this_attr.namespace_uri() != other_attr.namespace_uri())
            return false;
        if (this_attr.local_name() != other_attr.local_name())
            return false;
        if (this_attr.value() != other_attr.value())
            return false;
        break;
    }
    case (u16)NodeType::PROCESSING_INSTRUCTION_NODE: {
        // Its target and data.
        auto& this_processing_instruction = as<ProcessingInstruction>(*this);
        auto& other_processing_instruction = as<ProcessingInstruction>(*other_node);
        if (this_processing_instruction.target() != other_processing_instruction.target())
            return false;
        if (this_processing_instruction.data() != other_processing_instruction.data())
            return false;
        break;
    }
    default:
        break;
    }

    // A and B have the same number of children.
    if (child_count() != other_node->child_count())
        return false;

    // Each child of A equals the child of B at the identical index.
    auto* this_child = first_child();
    auto* other_child = other_node->first_child();
    while (this_child) {
        VERIFY(other_child);
        if (!this_child->is_equal_node(other_child))
            return false;

        this_child = this_child->next_sibling();
        other_child = other_child->next_sibling();
    }

    return true;
}

// https://dom.spec.whatwg.org/#locate-a-namespace
Optional<String> Node::locate_a_namespace(Optional<String> const& prefix) const
{
    // To locate a namespace for a node using prefix, switch on the interface node implements:

    // Element
    if (is<Element>(*this)) {
        // 1. If prefix is "xml", then return the XML namespace.
        if (prefix == "xml")
            return Web::Namespace::XML.to_string();

        // 2. If prefix is "xmlns", then return the XMLNS namespace.
        if (prefix == "xmlns")
            return Web::Namespace::XMLNS.to_string();

        // 3. If its namespace is non-null and its namespace prefix is prefix, then return namespace.
        auto& element = as<Element>(*this);
        if (element.namespace_uri().has_value() && element.prefix() == prefix)
            return element.namespace_uri()->to_string();

        // 4. If it has an attribute whose namespace is the XMLNS namespace, namespace prefix is "xmlns", and local name is prefix,
        //    or if prefix is null and it has an attribute whose namespace is the XMLNS namespace, namespace prefix is null,
        //    and local name is "xmlns", then return its value if it is not the empty string, and null otherwise.
        if (auto* attributes = element.attributes()) {
            for (size_t i = 0; i < attributes->length(); ++i) {
                auto& attr = *attributes->item(i);
                if (attr.namespace_uri() == Web::Namespace::XMLNS) {
                    if ((attr.prefix() == "xmlns" && attr.local_name() == prefix) || (!prefix.has_value() && !attr.prefix().has_value() && attr.local_name() == "xmlns")) {
                        auto value = attr.value();
                        if (!value.is_empty())
                            return value;

                        return {};
                    }
                }
            }
        }

        // 5. If its parent element is null, then return null.
        auto* parent_element = element.parent_element();
        if (!element.parent_element())
            return {};

        // 6. Return the result of running locate a namespace on its parent element using prefix.
        return parent_element->locate_a_namespace(prefix);
    }

    // Document
    if (is<Document>(*this)) {
        // 1. If its document element is null, then return null.
        auto* document_element = as<Document>(*this).document_element();
        if (!document_element)
            return {};

        // 2. Return the result of running locate a namespace on its document element using prefix.
        return document_element->locate_a_namespace(prefix);
    }

    // DocumentType
    // DocumentFragment
    if (is<DocumentType>(*this) || is<DocumentFragment>(*this)) {
        // Return null.
        return {};
    }

    // Attr
    if (is<Attr>(*this)) {
        // 1. If its element is null, then return null.
        auto* element = as<Attr>(*this).owner_element();
        if (!element)
            return {};

        // 2. Return the result of running locate a namespace on its element using prefix.
        return element->locate_a_namespace(prefix);
    }

    // Otherwise
    // 1. If its parent element is null, then return null.
    auto* parent_element = this->parent_element();
    if (!parent_element)
        return {};

    // 2. Return the result of running locate a namespace on its parent element using prefix.
    return parent_element->locate_a_namespace(prefix);
}

// https://dom.spec.whatwg.org/#dom-node-lookupnamespaceuri
Optional<String> Node::lookup_namespace_uri(Optional<String> prefix) const
{
    // 1. If prefix is the empty string, then set it to null.
    if (prefix.has_value() && prefix->is_empty())
        prefix = {};

    // 2. Return the result of running locate a namespace for this using prefix.
    return locate_a_namespace(prefix);
}

// https://dom.spec.whatwg.org/#dom-node-lookupprefix
Optional<String> Node::lookup_prefix(Optional<String> namespace_) const
{
    // 1. If namespace is null or the empty string, then return null.
    if (!namespace_.has_value() || namespace_->is_empty())
        return {};

    // 2. Switch on the interface this implements:

    // Element
    if (is<Element>(*this)) {
        // Return the result of locating a namespace prefix for it using namespace.
        auto& element = as<Element>(*this);
        return element.locate_a_namespace_prefix(namespace_);
    }

    // Document
    if (is<Document>(*this)) {
        // Return the result of locating a namespace prefix for its document element, if its document element is non-null; otherwise null.
        auto* document_element = as<Document>(*this).document_element();
        if (!document_element)
            return {};

        return document_element->locate_a_namespace_prefix(namespace_);
    }

    // DocumentType
    // DocumentFragment
    if (is<DocumentType>(*this) || is<DocumentFragment>(*this))
        // Return null
        return {};

    // Attr
    if (is<Attr>(*this)) {
        // Return the result of locating a namespace prefix for its element, if its element is non-null; otherwise null.
        auto* element = as<Attr>(*this).owner_element();
        if (!element)
            return {};

        return element->locate_a_namespace_prefix(namespace_);
    }

    // Otherwise
    // Return the result of locating a namespace prefix for its parent element, if its parent element is non-null; otherwise null.
    auto* parent_element = this->parent_element();
    if (!parent_element)
        return {};

    return parent_element->locate_a_namespace_prefix(namespace_);
}

// https://dom.spec.whatwg.org/#dom-node-isdefaultnamespace
bool Node::is_default_namespace(Optional<String> namespace_) const
{
    // 1. If namespace is the empty string, then set it to null.
    if (namespace_.has_value() && namespace_->is_empty())
        namespace_ = {};

    // 2. Let defaultNamespace be the result of running locate a namespace for this using null.
    auto default_namespace = locate_a_namespace({});

    // 3. Return true if defaultNamespace is the same as namespace; otherwise false.
    return default_namespace == namespace_;
}

bool Node::is_inert() const
{
    if (auto* html_element = as_if<HTML::HTMLElement>(*this))
        return html_element->is_inert();

    if (auto* enclosing_html_element = this->enclosing_html_element())
        return enclosing_html_element->is_inert();

    return false;
}

// https://dom.spec.whatwg.org/#in-a-document-tree
bool Node::in_a_document_tree() const
{
    // An element is in a document tree if its root is a document.
    return root().is_document();
}

// https://dom.spec.whatwg.org/#dom-node-getrootnode
GC::Ref<Node> Node::get_root_node(GetRootNodeOptions const& options)
{
    // The getRootNode(options) method steps are to return this’s shadow-including root if options["composed"] is true;
    if (options.composed)
        return shadow_including_root();

    // otherwise this’s root.
    return root();
}

String Node::debug_description() const
{
    StringBuilder builder;
    builder.append(node_name().to_deprecated_fly_string().to_lowercase());
    if (is_element()) {
        auto const& element = static_cast<DOM::Element const&>(*this);
        if (element.id().has_value())
            builder.appendff("#{}", element.id().value());
        for (auto const& class_name : element.class_names())
            builder.appendff(".{}", class_name);
    }
    return MUST(builder.to_string());
}

// https://dom.spec.whatwg.org/#concept-node-length
size_t Node::length() const
{
    // 1. If node is a DocumentType or Attr node, then return 0.
    if (is_document_type() || is_attribute())
        return 0;

    // 2. If node is a CharacterData node, then return node’s data’s length.
    if (is_character_data())
        return as<CharacterData>(*this).length_in_utf16_code_units();

    // 3. Return the number of node’s children.
    return child_count();
}

void Node::set_paintable(GC::Ptr<Painting::Paintable> paintable)
{
    m_paintable = paintable;
}

void Node::clear_paintable()
{
    m_paintable = nullptr;
}

Painting::Paintable const* Node::paintable() const
{
    return m_paintable;
}

Painting::Paintable* Node::paintable()
{
    return m_paintable;
}

Painting::PaintableBox const* Node::paintable_box() const
{
    if (paintable() && paintable()->is_paintable_box())
        return static_cast<Painting::PaintableBox const*>(paintable());
    return nullptr;
}

Painting::PaintableBox* Node::paintable_box()
{
    if (paintable() && paintable()->is_paintable_box())
        return static_cast<Painting::PaintableBox*>(paintable());
    return nullptr;
}

// https://dom.spec.whatwg.org/#queue-a-mutation-record
void Node::queue_mutation_record(FlyString const& type, Optional<FlyString> const& attribute_name, Optional<FlyString> const& attribute_namespace, Optional<String> const& old_value, Vector<GC::Root<Node>> added_nodes, Vector<GC::Root<Node>> removed_nodes, Node* previous_sibling, Node* next_sibling) const
{
    // NOTE: We defer garbage collection until the end of the scope, since we can't safely use MutationObserver* as a hashmap key otherwise.
    // FIXME: This is a total hack.
    GC::DeferGC defer_gc(heap());

    // 1. Let interestedObservers be an empty map.
    // mutationObserver -> mappedOldValue
    OrderedHashMap<MutationObserver*, Optional<String>> interested_observers;

    // 2. Let nodes be the inclusive ancestors of target.
    // 3. For each node in nodes, and then for each registered of node’s registered observer list:
    for (auto* node = this; node; node = node->parent()) {
        if (!node->m_registered_observer_list)
            continue;
        for (auto& registered_observer : *node->m_registered_observer_list) {
            // 1. Let options be registered’s options.
            auto& options = registered_observer->options();

            // 2. If none of the following are true
            //      - node is not target and options["subtree"] is false
            //      - type is "attributes" and options["attributes"] either does not exist or is false
            //      - type is "attributes", options["attributeFilter"] exists, and options["attributeFilter"] does not contain name or namespace is non-null
            //      - type is "characterData" and options["characterData"] either does not exist or is false
            //      - type is "childList" and options["childList"] is false
            //    then:
            if (!(node != this && !options.subtree)
                && !(type == MutationType::attributes && (!options.attributes.has_value() || !options.attributes.value()))
                && !(type == MutationType::attributes && options.attribute_filter.has_value() && (attribute_namespace.has_value() || !options.attribute_filter->contains_slow(attribute_name.value_or(String {}))))
                && !(type == MutationType::characterData && (!options.character_data.has_value() || !options.character_data.value()))
                && !(type == MutationType::childList && !options.child_list)) {
                // 1. Let mo be registered’s observer.
                auto mutation_observer = registered_observer->observer();

                // 2. If interestedObservers[mo] does not exist, then set interestedObservers[mo] to null.
                if (!interested_observers.contains(mutation_observer))
                    interested_observers.set(mutation_observer, {});

                // 3. If either type is "attributes" and options["attributeOldValue"] is true, or type is "characterData" and options["characterDataOldValue"] is true, then set interestedObservers[mo] to oldValue.
                if ((type == MutationType::attributes && options.attribute_old_value.has_value() && options.attribute_old_value.value()) || (type == MutationType::characterData && options.character_data_old_value.has_value() && options.character_data_old_value.value()))
                    interested_observers.set(mutation_observer, old_value);
            }
        }
    }

    // OPTIMIZATION: If there are no interested observers, bail without doing any more work.
    if (interested_observers.is_empty())
        return;

    auto added_nodes_list = StaticNodeList::create(realm(), move(added_nodes));
    auto removed_nodes_list = StaticNodeList::create(realm(), move(removed_nodes));

    // 4. For each observer → mappedOldValue of interestedObservers:
    for (auto& interested_observer : interested_observers) {
        // FIXME: The MutationRecord constructor shuld take an Optional<FlyString> attribute name and namespace
        Optional<String> string_attribute_name;
        if (attribute_name.has_value())
            string_attribute_name = attribute_name->to_string();
        Optional<String> string_attribute_namespace;
        if (attribute_namespace.has_value())
            string_attribute_name = attribute_namespace->to_string();

        // 1. Let record be a new MutationRecord object with its type set to type, target set to target, attributeName set to name, attributeNamespace set to namespace, oldValue set to mappedOldValue,
        //    addedNodes set to addedNodes, removedNodes set to removedNodes, previousSibling set to previousSibling, and nextSibling set to nextSibling.
        auto record = MutationRecord::create(realm(), type, *this, added_nodes_list, removed_nodes_list, previous_sibling, next_sibling, string_attribute_name, string_attribute_namespace, /* mappedOldValue */ interested_observer.value);

        // 2. Enqueue record to observer’s record queue.
        interested_observer.key->enqueue_record({}, move(record));
    }

    // 5. Queue a mutation observer microtask.
    Bindings::queue_mutation_observer_microtask(document());
}

// https://dom.spec.whatwg.org/#queue-a-tree-mutation-record
void Node::queue_tree_mutation_record(Vector<GC::Root<Node>> added_nodes, Vector<GC::Root<Node>> removed_nodes, Node* previous_sibling, Node* next_sibling)
{
    // 1. Assert: either addedNodes or removedNodes is not empty.
    VERIFY(added_nodes.size() > 0 || removed_nodes.size() > 0);

    // 2. Queue a mutation record of "childList" for target with null, null, null, addedNodes, removedNodes, previousSibling, and nextSibling.
    queue_mutation_record(MutationType::childList, {}, {}, {}, move(added_nodes), move(removed_nodes), previous_sibling, next_sibling);
}

void Node::append_child_impl(GC::Ref<Node> node)
{
    VERIFY(!node->parent());

    if (!is_child_allowed(*node))
        return;

    TreeNode::append_child(node);
}

void Node::insert_before_impl(GC::Ref<Node> node, GC::Ptr<Node> child)
{
    if (!child)
        return append_child_impl(move(node));
    TreeNode::insert_before(node, child);
}

void Node::remove_child_impl(GC::Ref<Node> node)
{
    TreeNode::remove_child(node);
}

bool Node::is_descendant_of(Node const& other) const
{
    return other.is_ancestor_of(*this);
}

bool Node::is_inclusive_descendant_of(Node const& other) const
{
    return other.is_inclusive_ancestor_of(*this);
}

// https://dom.spec.whatwg.org/#concept-tree-following
bool Node::is_following(Node const& other) const
{
    // An object A is following an object B if A and B are in the same tree and A comes after B in tree order.
    for (auto* node = previous_in_pre_order(); node; node = node->previous_in_pre_order()) {
        if (node == &other)
            return true;
    }

    return false;
}

void Node::build_accessibility_tree(AccessibilityTreeNode& parent)
{
    if (is_uninteresting_whitespace_node())
        return;

    if (is_document()) {
        auto* document = static_cast<DOM::Document*>(this);
        auto* document_element = document->document_element();
        if (document_element && document_element->include_in_accessibility_tree()) {
            parent.set_value(document_element);
            if (document_element->has_child_nodes())
                document_element->for_each_child([&parent](DOM::Node& child) {
                    child.build_accessibility_tree(parent);
                    return IterationDecision::Continue;
                });
        }
    } else if (is_element()) {
        auto const* element = static_cast<DOM::Element const*>(this);

        if (is<HTML::HTMLScriptElement>(element) || is<HTML::HTMLStyleElement>(element))
            return;

        if (element->include_in_accessibility_tree()) {
            auto current_node = AccessibilityTreeNode::create(&document(), this);
            parent.append_child(current_node);
            if (has_child_nodes()) {
                for_each_child([&current_node](DOM::Node& child) {
                    child.build_accessibility_tree(*current_node);
                    return IterationDecision::Continue;
                });
            }
        } else if (has_child_nodes()) {
            for_each_child([&parent](DOM::Node& child) {
                child.build_accessibility_tree(parent);
                return IterationDecision::Continue;
            });
        }
    } else if (is_text()) {
        parent.append_child(AccessibilityTreeNode::create(&document(), this));
        if (has_child_nodes()) {
            for_each_child([&parent](DOM::Node& child) {
                child.build_accessibility_tree(parent);
                return IterationDecision::Continue;
            });
        }
    }
}

// https://www.w3.org/TR/accname-1.2/#mapping_additional_nd_te
ErrorOr<String> Node::name_or_description(NameOrDescription target, Document const& document, HashTable<UniqueNodeID>& visited_nodes, IsDescendant is_descendant, ShouldComputeRole should_compute_role) const
{
    // The text alternative for a given element is computed as follows:
    // 1. Set the root node to the given element, the current node to the root node, and the total accumulated text to the
    //    empty string (""). If the root node's role prohibits naming, return the empty string ("").
    auto const* root_node = this;
    auto const* current_node = root_node;
    StringBuilder total_accumulated_text;
    visited_nodes.set(unique_id());

    if (is_element()) {
        auto const* element = static_cast<DOM::Element const*>(this);
        Optional<ARIA::Role> role = OptionalNone {};
        // Per https://w3c.github.io/aria/#document-handling_author-errors_roles, determining whether to ignore certain
        // specified landmark roles requires first determining, in the ARIAMixin code, whether the element for which the
        // role is specified has an accessible name — that is, calling into this name_or_description code. But if we
        // then try to retrieve a role for such elements here, that’d then end up calling right back into this
        // name_or_description code — which would cause the calls to loop infinitely. So to avoid that, the caller
        // in the ARIAMixin code can pass the shouldComputeRole parameter to indicate we must skip the role lookup.
        if (should_compute_role == ShouldComputeRole::Yes)
            role = element->role_from_role_attribute_value();
        // Per https://w3c.github.io/html-aam/#el-aside and https://w3c.github.io/html-aam/#el-section, computing a
        // default role for an aside element or section element requires first computing its accessible name — that is,
        // calling into this name_or_description code. But if we then try to determine a default role for the aside
        // element or section element here, that’d then end up calling right back into this name_or_description code —
        // which would cause the calls to loop infinitely. So to avoid that, we only compute a default role here if this
        // isn’t an aside element or section element.
        // https://github.com/w3c/aria/issues/2391
        if (!role.has_value() && element->local_name() != HTML::TagNames::aside && element->local_name() != HTML::TagNames::section)
            role = element->default_role();

        // 2. Compute the text alternative for the current node:

        // A. Hidden Not Referenced: If the current node is hidden and is:
        // i. Not part of an aria-labelledby or aria-describedby traversal, where the node directly referenced by that
        //    relation was hidden.
        // ii. Nor part of a native host language text alternative element (e.g. label in HTML) or attribute traversal,
        //     where the root of that traversal was hidden.
        // Return the empty string.
        //
        // NOTE: Nodes with CSS properties display:none, visibility:hidden, visibility:collapse or content-visibility:hidden:
        //       They are considered hidden, as they match the guidelines "not perceivable" and "explicitly hidden".
        //
        // AD-HOC: We don’t implement this step here — because strictly implementing this would cause us to return early
        // whenever encountering a node (element, actually) that “is hidden and is not directly referenced by
        // aria-labelledby or aria-describedby”, without traversing down through that element’s subtree to see if it has
        // (1) any descendant elements that are directly referenced and/or (2) any un-hidden nodes. So we instead (in
        // substep G below) traverse upward through ancestor nodes of every text node, and check in that way to do the
        // equivalent of what this step seems to have been intended to do.
        // https://github.com/w3c/aria/issues/2387

        // B. Otherwise:
        // - if computing a name, and the current node has an aria-labelledby attribute that contains at least one valid
        //   IDREF, and the current node is not already part of an aria-labelledby traversal, process its IDREFs in the
        //   order they occur:
        // - or, if computing a description, and the current node has an aria-describedby attribute that contains at least
        //   one valid IDREF, and the current node is not already part of an aria-describedby traversal, process its IDREFs
        //   in the order they occur:
        auto aria_labelled_by = element->aria_labelled_by();
        auto aria_described_by = element->aria_described_by();

        if ((target == NameOrDescription::Name && aria_labelled_by.has_value() && Node::first_valid_id(*aria_labelled_by, document).has_value())
            || (target == NameOrDescription::Description && aria_described_by.has_value() && Node::first_valid_id(*aria_described_by, document).has_value())) {

            // i. Set the accumulated text to the empty string.
            total_accumulated_text.clear();

            Vector<StringView> id_list;
            if (target == NameOrDescription::Name) {
                id_list = aria_labelled_by->bytes_as_string_view().split_view_if(Infra::is_ascii_whitespace);
            } else {
                id_list = aria_described_by->bytes_as_string_view().split_view_if(Infra::is_ascii_whitespace);
            }

            // ii. For each IDREF:
            for (auto const& id_ref : id_list) {
                auto node = document.get_element_by_id(MUST(FlyString::from_utf8(id_ref)));
                if (!node)
                    continue;
                // AD-HOC: The “For each IDREF” substep in the spec doesn’t seem to explicitly require the following
                // check for an aria-label value; but the “div group explicitly labelledby self and heading” subtest at
                // https://wpt.fyi/results/accname/name/comp_labelledby.html won’t pass unless we do this check.
                // https://github.com/w3c/aria/issues/2388
                if (target == NameOrDescription::Name && node->aria_label().has_value() && !node->aria_label()->is_empty() && !node->aria_label()->bytes_as_string_view().is_whitespace()) {
                    total_accumulated_text.append(' ');
                    total_accumulated_text.append(node->aria_label().value());
                }
                if (visited_nodes.contains(node->unique_id()))
                    continue;

                // a. Set the current node to the node referenced by the IDREF.
                current_node = node;
                // b. Compute the text alternative of the current node beginning with step 2. Set the result to that text alternative.
                auto result = TRY(node->name_or_description(target, document, visited_nodes));
                // c. Append the result, with a space, to the accumulated text.
                total_accumulated_text.append(' ');
                total_accumulated_text.append(result);
            }

            // iii. Return the accumulated text.
            // AD-HOC: This substep in the spec doesn’t seem to explicitly require the following check for an aria-label
            // value; but the “button's hidden referenced name (visibility:hidden) with hidden aria-labelledby traversal
            // falls back to aria-label” subtest at https://wpt.fyi/results/accname/name/comp_labelledby.html won’t pass
            // unless we do this check.
            // https://github.com/w3c/aria/issues/2388
            if (total_accumulated_text.string_view().is_whitespace() && target == NameOrDescription::Name && element->aria_label().has_value() && !element->aria_label()->is_empty() && !element->aria_label()->bytes_as_string_view().is_whitespace())
                return element->aria_label().release_value();
            return total_accumulated_text.to_string();
        }

        // D. AriaLabel: Otherwise, if the current node has an aria-label attribute whose value is not undefined, not
        //    the empty string, nor, when trimmed of whitespace, is not the empty string:
        //
        // AD-HOC: We’ve reordered substeps C and D from https://w3c.github.io/accname/#step2 — because
        // the more-specific per-HTML-element requirements at https://w3c.github.io/html-aam/#accname-computation
        // necessitate doing so, and the “input with label for association is superceded by aria-label” subtest at
        // https://wpt.fyi/results/accname/name/comp_label.html won’t pass unless we do this reordering.
        // Spec PR: https://github.com/w3c/aria/pull/2377
        if (target == NameOrDescription::Name && element->aria_label().has_value() && !element->aria_label()->is_empty() && !element->aria_label()->bytes_as_string_view().is_whitespace()) {
            // TODO: - If traversal of the current node is due to recursion and the current node is an embedded control as defined in step 2E, ignore aria-label and skip to rule 2E.
            // https://github.com/w3c/aria/pull/2385 and https://github.com/w3c/accname/issues/173
            if (!element->is_html_slot_element())
                return element->aria_label().value();
        }

        // C. Embedded Control: Otherwise, if the current node is a control embedded within the label (e.g. any element
        //    directly referenced by aria-labelledby) for another widget, where the user can adjust the embedded control's
        //    value, then return the embedded control as part of the text alternative in the following manner:
        GC::Ptr<DOM::NodeList> labels;
        if (is<HTML::HTMLElement>(this))
            labels = (const_cast<HTML::HTMLElement&>(static_cast<HTML::HTMLElement const&>(*current_node))).labels();
        if (labels != nullptr && labels->length() > 0) {
            StringBuilder builder;
            for (u32 i = 0; i < labels->length(); i++) {
                if (!builder.is_empty())
                    builder.append(" "sv);
                auto nodes = labels->item(i)->children_as_vector();
                for (auto const& node : nodes) {
                    // AD-HOC: https://wpt.fyi/results/accname/name/comp_host_language_label.html has “encapsulation”
                    // tests, from which can be induced a requirement that when computing the accessible name for a
                    // <label>-ed form control (“embedded control”), then any content (text content or attribute values)
                    // from the control itself that would otherwise be included in the accessible-name computation for
                    // it ancestor <label> must instead be skipped and not included. The HTML-AAM spec seems to maybe
                    // be trying to achieve that result by expressing specific steps for each particular type of form
                    // control. But what all that reduces/optimizes/simplifies down to is just, “skip over self”.
                    // https://github.com/w3c/aria/issues/2389
                    if (node == this)
                        continue;

                    if (node->is_element()) {
                        auto const& element = static_cast<DOM::Element const&>(*node);
                        auto role = element.role_or_default();

                        if (role == ARIA::Role::textbox) {
                            // i. Textbox: If the embedded control has role textbox, return its value.
                            if (is<HTML::HTMLInputElement>(*node)) {
                                auto const& element = static_cast<HTML::HTMLInputElement const&>(*node);
                                if (element.has_attribute(HTML::AttributeNames::value))
                                    builder.append(element.value());
                            } else
                                builder.append(node->text_content().value());
                        } else if (role == ARIA::Role::combobox) {
                            // ii. Combobox/Listbox: If the embedded control has role combobox or listbox, return the text
                            //     alternative of the chosen option.
                            if (is<HTML::HTMLInputElement>(*node)) {
                                auto const& element = static_cast<HTML::HTMLInputElement const&>(*node);
                                if (element.has_attribute(HTML::AttributeNames::value))
                                    builder.append(element.value());
                            } else if (is<HTML::HTMLSelectElement>(*node)) {
                                auto const& element = static_cast<HTML::HTMLSelectElement const&>(*node);
                                builder.append(element.value());
                            } else
                                builder.append(node->text_content().value());
                        } else if (role == ARIA::Role::listbox) {
                            // ii. Combobox/Listbox: If the embedded control has role combobox or listbox, return the text
                            //     alternative of the chosen option.
                            if (is<HTML::HTMLSelectElement>(*node)) {
                                auto const& element = static_cast<HTML::HTMLSelectElement const&>(*node);
                                builder.append(element.value());
                            }
                            auto children = node->children_as_vector();
                            for (auto& child : children) {
                                if (child->is_element()) {
                                    auto const& element = static_cast<DOM::Element const&>(*child);
                                    auto role = element.role_or_default();
                                    if (role == ARIA::Role::option && element.aria_selected() == "true")
                                        builder.append(element.text_content().value());
                                }
                            }
                        } else if (role == ARIA::Role::spinbutton || role == ARIA::Role::slider) {
                            auto aria_valuenow = element.aria_value_now();
                            auto aria_valuetext = element.aria_value_text();

                            // iii. Range: If the embedded control has role range (e.g., a spinbutton or slider):
                            // a. If the aria-valuetext property is present, return its value,
                            if (aria_valuetext.has_value())
                                builder.append(aria_valuetext.value());
                            // b. Otherwise, if the aria-valuenow property is present, return its value
                            else if (aria_valuenow.has_value())
                                builder.append(aria_valuenow.value());
                            // c. Otherwise, use the value as specified by a host language attribute.
                            else if (is<HTML::HTMLInputElement>(*node)) {
                                auto const& element = static_cast<HTML::HTMLInputElement const&>(*node);
                                if (element.has_attribute(HTML::AttributeNames::value))
                                    builder.append(element.value());
                            }
                        }
                    } else if (node->is_text()) {
                        auto const& text_node = static_cast<DOM::Text const&>(*node);
                        builder.append(text_node.data());
                    }
                }
            }
            return builder.to_string();
        }

        // E. Host Language Label: Otherwise, if the current node's native markup provides an attribute (e.g. alt) or
        //    element (e.g. HTML label or SVG title) that defines a text alternative, return that alternative in the form
        //    of a flat string as defined by the host language.
        // TODO: Confirm (through existing WPT test cases) whether HTMLLabelElement is already handled (by the code for
        // step C. “Embedded Control” above) in conformance with the spec requirements — and if not, then add handling.
        //
        // https://w3c.github.io/html-aam/#img-element-accessible-name-computation
        // use alt attribute, even if its value is the empty string.
        // See also https://wpt.fyi/results/accname/name/comp_tooltip.tentative.html.
        if (is<HTML::HTMLImageElement>(*element) && element->has_attribute(HTML::AttributeNames::alt))
            return element->get_attribute(HTML::AttributeNames::alt).value();

        // https://w3c.github.io/svg-aam/#mapping_additional_nd
        Optional<String> title_element_text;
        if (element->is_svg_element()) {
            // If the current node has at least one direct child title element, select the appropriate title based on
            // the language rules for the SVG specification, and return the title text alternative as a flat string.
            element->for_each_child_of_type<SVG::SVGTitleElement>([&](SVG::SVGTitleElement const& title) mutable {
                title_element_text = title.text_content();
                return IterationDecision::Break;
            });
            if (title_element_text.has_value())
                return title_element_text.release_value();

            // If the current node is a link, and there was no child title element, but it has an xlink:title attribute,
            // return the value of that attribute.
            if (auto title_attribute = element->get_attribute_ns(Namespace::XLink, XLink::AttributeNames::title); title_attribute.has_value())
                return title_attribute.release_value();
        }

        // https://w3c.github.io/html-aam/#table-element-accessible-name-computation
        // 2. If the accessible name is still empty, then: if the table element has a child that is a caption element,
        //    then use the subtree of the first such element.
        if (is<HTML::HTMLTableElement>(*element))
            if (auto& table = (const_cast<HTML::HTMLTableElement&>(static_cast<HTML::HTMLTableElement const&>(*element))); table.caption())
                return table.caption()->text_content().release_value();

        // https://w3c.github.io/html-aam/#fieldset-element-accessible-name-computation
        // 2. If the accessible name is still empty, then: if the fieldset element has a child that is a legend element,
        //    then use the subtree of the first such element.
        if (is<HTML::HTMLFieldSetElement>(*element)) {
            Optional<String> legend;
            auto& fieldset = (const_cast<HTML::HTMLFieldSetElement&>(static_cast<HTML::HTMLFieldSetElement const&>(*element)));
            fieldset.for_each_child_of_type<HTML::HTMLLegendElement>([&](HTML::HTMLLegendElement const& element) mutable {
                legend = element.text_content().release_value();
                return IterationDecision::Break;
            });
            if (legend.has_value())
                return legend.release_value();
        }

        if (is<HTML::HTMLInputElement>(*element)) {
            auto& input = (const_cast<HTML::HTMLInputElement&>(static_cast<HTML::HTMLInputElement const&>(*element)));
            // https://w3c.github.io/html-aam/#input-type-button-input-type-submit-and-input-type-reset-accessible-name-computation
            // 3. Otherwise use the value attribute.
            if (input.type_state() == HTML::HTMLInputElement::TypeAttributeState::Button
                || input.type_state() == HTML::HTMLInputElement::TypeAttributeState::SubmitButton
                || input.type_state() == HTML::HTMLInputElement::TypeAttributeState::ResetButton)
                if (auto value = input.get_attribute(HTML::AttributeNames::value); value.has_value())
                    return value.release_value();

            // https://w3c.github.io/html-aam/#input-type-image-accessible-name-computation
            // 3. Otherwise use alt attribute if present and its value is not the empty string.
            if (input.type_state() == HTML::HTMLInputElement::TypeAttributeState::ImageButton)
                if (auto alt = element->get_attribute(HTML::AttributeNames::alt); alt.has_value())
                    return alt.release_value();
        }

        // F. Name From Content: Otherwise, if the current node's role allows name from content, or if the current node
        //    is referenced by aria-labelledby, aria-describedby, or is a native host language text alternative element
        //    (e.g. label in HTML), or is a descendant of a native host language text alternative element:
        if ((role.has_value() && ARIA::allows_name_from_content(role.value())) || element->is_referenced() || is_descendant == IsDescendant::Yes) {
            // i. Set the accumulated text to the empty string.
            total_accumulated_text.clear();

            // ii. Name From Generated Content: Check for CSS generated textual content associated with the current node
            //     and include it in the accumulated text. The CSS ::before and ::after pseudo elements [CSS2] can provide
            //     textual content for elements that have a content model.
            // a. For ::before pseudo elements, User agents MUST prepend CSS textual content, without a space, to the textual
            //    content of the current node.
            // b. For ::after pseudo elements, User agents MUST append CSS textual content, without a space, to the textual
            //    content of the current node. NOTE: The code for handling the ::after pseudo elements case is further below,
            //    following the “iii. For each child node of the current node” code.
            if (auto before = element->get_pseudo_element_node(CSS::Selector::PseudoElement::Type::Before)) {
                if (before->computed_values().content().alt_text.has_value())
                    total_accumulated_text.append(before->computed_values().content().alt_text.release_value());
                else
                    total_accumulated_text.append(before->computed_values().content().data);
            }

            // iii. Determine Child Nodes: Determine the rendered child nodes of the current node:
            // iii. Determine Child Nodes: Determine the rendered child nodes of the current node:
            // c. [Otherwise,] set the rendered child nodes to be the child nodes of the current node.
            auto child_nodes = current_node->children_as_vector();

            // a. If the current node has an attached shadow root, set the rendered child nodes to be the child nodes of
            //    the shadow root.
            if (element->is_shadow_host() && element->shadow_root() && element->shadow_root()->is_connected())
                child_nodes = element->shadow_root()->children_as_vector();

            // b. Otherwise, if the current node is a slot with assigned nodes, set the rendered child nodes to be the
            //    assigned nodes of the current node.
            if (element->is_html_slot_element()) {
                total_accumulated_text.append(element->text_content().value());
                child_nodes = static_cast<HTML::HTMLSlotElement const*>(element)->assigned_nodes();
            }

            // iv. Name From Each Child: For each rendered child node of the current node
            for (auto& child_node : child_nodes) {
                if (!child_node->is_element() && !child_node->is_text())
                    continue;
                bool should_add_space = true;
                const_cast<DOM::Document&>(document).update_layout();
                auto const* layout_node = child_node->layout_node();
                if (layout_node) {
                    auto display = layout_node->display();
                    if (display.is_inline_outside() && display.is_flow_inside()) {
                        should_add_space = false;
                    }
                }
                if (visited_nodes.contains(child_node->unique_id()))
                    continue;

                // a. Set the current node to the child node.
                current_node = child_node;

                // b. Compute the text alternative of the current node beginning with step 2. Set the result to that text alternative.
                auto result = MUST(current_node->name_or_description(target, document, visited_nodes, IsDescendant::Yes, should_compute_role));

                // J. Append a space character and the result of each step above to the total accumulated text.
                // AD-HOC: Doing the space-adding here is in a different order from what the spec states.
                if (should_add_space)
                    total_accumulated_text.append(' ');

                // c. Append the result to the accumulated text.
                total_accumulated_text.append(result);
            }

            // NOTE: See step ii.b above.
            if (auto after = element->get_pseudo_element_node(CSS::Selector::PseudoElement::Type::After)) {
                if (after->computed_values().content().alt_text.has_value())
                    total_accumulated_text.append(after->computed_values().content().alt_text.release_value());
                else
                    total_accumulated_text.append(after->computed_values().content().data);
            }

            // v. Return the accumulated text if it is not the empty string ("").
            if (!total_accumulated_text.is_empty())
                return total_accumulated_text.to_string();

            // Important: Each node in the subtree is consulted only once. If text has been collected from a descendant,
            // but is referenced by another IDREF in some descendant node, then that second, or subsequent, reference is
            // not followed. This is done to avoid infinite loops.
        }
    }

    // G. Text Node: Otherwise, if the current node is a Text Node, return its textual contents.
    //
    // AD-HOC: The spec doesn’t require ascending through the parent node and ancestor nodes of every text node we
    // reach — the way we’re doing there. But we implement it this way because the spec algorithm as written doesn’t
    // appear to achieve what it seems to be intended to achieve. Specifically, the spec algorithm as written doesn’t
    // cause traversal through element subtrees in way that’s necessary to check for descendants that are referenced by
    // aria-labelledby or aria-describedby and/or un-hidden. See the comment for substep A above.
    if (is_text() && (!parent_element() || (parent_element()->is_referenced() || !parent_element()->is_hidden() || !parent_element()->has_hidden_ancestor() || parent_element()->has_referenced_and_hidden_ancestor()))) {
        if (layout_node() && layout_node()->is_text_node())
            return as<Layout::TextNode>(layout_node())->text_for_rendering();
        return text_content().release_value();
    }

    // H. Otherwise, if the current node is a descendant of an element whose Accessible Name or Accessible Description
    //    is being computed, and contains descendants, proceed to 2F.i.
    //
    // AD-HOC: We don’t implement this step here — because is essentially unreachable code in the spec algorithm.
    // We could never get here without descending through every subtree of an element whose Accessible Name or
    // Accessible Description is being computed. And in our implementation of substep F about, we’re anyway already
    // recursively descending through all the child nodes of every element whose Accessible Name or Accessible
    // Description is being computed, in a way that never leads to this substep H every being hit.

    // I. Otherwise, if the current node has a Tooltip attribute, return its value.
    //
    // https://www.w3.org/TR/accname-1.2/#dfn-tooltip-attribute
    // Any host language attribute that would result in a user agent generating a tooltip such as in response to a mouse
    // hover in desktop user agents.
    // FIXME: Support SVG tooltips and CSS tooltips
    if (is<HTML::HTMLElement>(this)) {
        auto const* element = static_cast<HTML::HTMLElement const*>(this);
        auto tooltip = element->title();
        if (tooltip.has_value() && !tooltip->is_empty())
            return tooltip.release_value();
    }

    // 3. After all steps are completed, the total accumulated text is used as the accessible name or accessible description
    //    of the element that initiated the computation.
    return total_accumulated_text.to_string();
}

// https://www.w3.org/TR/accname-1.2/#mapping_additional_nd_name
ErrorOr<String> Node::accessible_name(Document const& document, ShouldComputeRole should_compute_role) const
{
    HashTable<UniqueNodeID> visited_nodes;
    // User agents MUST compute an accessible name using the rules outlined below in the section titled Accessible Name and Description Computation.
    return name_or_description(NameOrDescription::Name, document, visited_nodes, IsDescendant::No, should_compute_role);
}

// https://www.w3.org/TR/accname-1.2/#mapping_additional_nd_description
ErrorOr<String> Node::accessible_description(Document const& document) const
{
    // If aria-describedby is present, user agents MUST compute the accessible description by concatenating the text alternatives for elements referenced by an aria-describedby attribute on the current element.
    // The text alternatives for the referenced elements are computed using a number of methods, outlined below in the section titled Accessible Name and Description Computation.
    if (!is_element())
        return String {};

    auto const* element = static_cast<Element const*>(this);
    auto described_by = element->aria_described_by();
    if (!described_by.has_value())
        return String {};

    HashTable<UniqueNodeID> visited_nodes;
    StringBuilder builder;
    auto id_list = described_by->bytes_as_string_view().split_view_if(Infra::is_ascii_whitespace);
    for (auto const& id : id_list) {
        if (auto description_element = document.get_element_by_id(MUST(FlyString::from_utf8(id)))) {
            auto description = TRY(
                description_element->name_or_description(NameOrDescription::Description, document,
                    visited_nodes));
            if (!description.is_empty()) {
                if (builder.is_empty()) {
                    builder.append(description);
                } else {
                    builder.append(" "sv);
                    builder.append(description);
                }
            }
        }
    }
    return builder.to_string();
}

Optional<StringView> Node::first_valid_id(StringView value, Document const& document)
{
    auto id_list = value.split_view_if(Infra::is_ascii_whitespace);
    for (auto const& id : id_list) {
        if (document.get_element_by_id(MUST(FlyString::from_utf8(id))))
            return id;
    }
    return {};
}

void Node::add_registered_observer(RegisteredObserver& registered_observer)
{
    if (!m_registered_observer_list)
        m_registered_observer_list = make<Vector<GC::Ref<RegisteredObserver>>>();
    m_registered_observer_list->append(registered_observer);
}

}

namespace IPC {

template<>
ErrorOr<void> encode(Encoder& encoder, Web::UniqueNodeID const& value)
{
    return encode(encoder, value.value());
}

template<>
ErrorOr<Web::UniqueNodeID> decode(Decoder& decoder)
{
    auto value = TRY(decoder.decode<i64>());
    return Web::UniqueNodeID(value);
}

}
