property previousBox

This property returns the previous box in the DOM. Unlike many other navigation properties this walks the entire DOM, not just children or siblings. This property can be used to walk from the bottom of the DOM (or any starting point) upwards toward the root box.

function PrintTagsInDOM(startBox, reverse)
{
    if (startBox)
    {
        var box = startBox;
        while (box)
        {
            shell.print("Tag: " + box.tag);
            if (reverse)
                box = box.previousBox;
            else
                box = box.nextBox;
        }
    }
}