Source/core/dom/ParentNode.idl

[
    NoInterfaceObject
] interface ParentNode {
    [PerWorldBindings] readonly attribute HTMLCollection children;
    [PerWorldBindings] readonly attribute Element firstElementChild;
    [PerWorldBindings] readonly attribute Element lastElementChild;
    [PerWorldBindings] readonly attribute unsigned long childElementCount;
};

Document implements ParentNode;
DocumentFragment implements ParentNode;
Element implements ParentNode;