关于原生js获取的问题

<script>
var oBox = document.getElementById("box");（用这个方法是兼容的）
var aUl = document.getElementsByTagName("ul");//如果用oBox.getElementByTagName("ul")会有兼容性问题（IE6,7,8并不支持）
var aImg = aUl[1].getElementsByTagName("li");

var timer = play = null;
var i = index = 0;


function autoPlay ()
{
play = setInterval(function () {
index++;
index >= aImg.length && (index = 0);
show(index);


},2000);
}
autoPlay();//应用

//图片切换, 淡入淡出效果
function show (a)
{
index = a;
var alpha = 0;


clearInterval(timer);

for (i = 0; i < aImg.length; i++)
{
aImg[i].style.opacity = 0;
aImg[i].style.filter = "alpha(opacity=0)";
}

timer = setInterval(function () {
alpha += 2;
alpha > 100 && (alpha =100);
aImg[index].style.opacity = alpha / 100;
aImg[index].style.filter = "alpha(opacity = " + alpha + ")";
alpha == 100 && clearInterval(timer)
},20);
}


</script>