不知道为什么拖动没有反应，求大神解答

Question

// JavaScript Document

function getByClass(ClassName,parent){

var oParent=parent?document.getElementById(parent):document,

eles=[],

elements=oParent.getElementsByTagName("*");

for(var i=0,l=elements.length;i<l;i++){

if(elements[i].className=ClassName){

eles.push(elements[i]);

}

}

return eles;

}

window.onload=drag;

function drag(){

var oTitle=document.getElementsByTagName("login_logo_webqq","loginPanel")[0];

oTitle.onmousedown=fnDown();

}

function fnDown(){

var oDrag = document.getElementById("loginPanel");

document.onmousemove=function(event){

event=event||window.event;

oDrag.style.left=clientX+"px";

oDrag.style.top=clientY+"px";

}

}