appendChild method
Implementation
@mustCallSuper
Node appendChild(Node child) {
child._ensureOrphan();
child.parentNode = this;
childNodes.add(child);
// To insert a node into a parent before a child, run step 9 from the spec:
// 9. Run the children changed steps for parent when inserting a node into a parent.
// https://dom.spec.whatwg.org/#concept-node-insert
childrenChanged();
if (child.isConnected) {
child.connectedCallback();
}
return child;
}