var z=document.getElementsByTagName("li")[4];

 function get_previousSibling(n)
    {
        var z=n.previousSibling;
        while (z&&z.nodeType!=1)
        {
            z=z.previousSibling;
        }
        return z;
    }
   var z=document.getElementsByTagName("li")[4];
       document.write(z.nodeName+" = "+z.innerHTML);
   var d=get_previousSibling(z);

为什么是【4】