The Node.firstChild read-only property returns the node's first child in the tree, or null if the node is childless. If the node is a Document, it returns the first node in the list of its direct children.
The Node.isEqualNode() method tests whether two nodes are equal. Two nodes are equal when they have the same type, defining characteristics (for elements, this would be their ID, number of children, and so forth), its attributes match, and so on. The specific set of data points that much match varies depending on the types of the nodes.
The ChildNode.after() method inserts a set of Node or DOMString objects in the children list of this ChildNode's parent, just after this ChildNode. DOMString objects are inserted as equivalent Text nodes.
The ChildNode.before method inserts a set of Node or DOMString objects in the children list of this ChildNode's parent, just before this ChildNode. DOMString objects are inserted as equivalent Text nodes.
The Node.appendChild() method adds a node to the end of the list of children of a specified parent node. If the given child is a reference to an existing node in the document, appendChild() moves it from its current position to the new position (there is no requirement to remove the node from its parent node before appending it to some other node).
The Node.lookupNamespaceURI() method takes a prefix and returns the namespace URI associated with it on the given node if found (and null if not). Supplying null for the prefix will return the default namespace.