The ElementTraversal interface is a set of read-only attributes
which allow an author to easily navigate between elements in a document.
In conforming implementations of Element Traversal, all objects that
implement Element must also implement the ElementTraversal
interface. Four of the methods,
getFirstElementChild, getLastElementChild,
getPreviousElementSibling, and getNextElementSibling,
each provides a live reference to another element with the defined
relationship to the current element, if the related element exists. The
fifth method, getChildElementCount, exposes the number of child
elements of an element, for preprocessing before navigation.
Element Traversal Specification