walking the DOM from Douglas Crockford, JavaScript: The Good Parts, p. 36

walkingTheDOM.js

/** Define a walk_the_DOM function that visits every
 * node of the tree in HTML source order, starting
 * from some given node. It invokes a function,
 * passing it each node in turn. walk_the_DOM calls
 * itself to process each of the child nodes.
*/
var walk_the_DOM = function walk(node, callback) {
    callback(node);
    node = node.firstChild;
    while (node) {
        walk(node, callback);
        node = node.nextSibling;
    }
};