js加上不class元素

[myron4048]

《html》代码如下：
<div class="payl">
<div id="stab1" class="tab-down mb clearfix">
<div class="playfrom tab8 clearfix">
<ul id="tab22">
<li id="tab82" onclick="settab('tab8','stab8',2,2)"><i class="playerico ico-Azhan"></i>2.测试</li>
</ul>
<ul id="tab21">
<li id="tab81" onclick="settab('tab8','stab8',1,2)"><i class="playerico ico-Azhan"></i>1.测试</li>
</ul>
</div>
<div id="stab82" class="playlist clearfix">
<div class="h1 clearfix">
<p class="intro"> </p>
<p class="jj"> <span id="vlink_2_s1"><em onclick="Order(1,'vlink_2',2)">倒序↓</em></span><span id="vlink_2_s2"><em onclick="Order(0,'vlink_2',2)" class="over">顺序↑</em></span> </p>
</div>
<div id="vlink_2" class="videourl clearfix">
<ul>
<li><a title='1' href='/' target="_self">1</a></li>
<li><a title='2' href='/' style="color:red" target="_self">2</a></li>
<li><a title='3' href='/' target="_self">3</a></li>
<li><a title='4' href='/' target="_self">4</a></li>
</ul>
</div>
</div>
<div id="stab81" class="playlist clearfix" >
<div class="h1 clearfix">
<p class="intro"> </p>
<p class="jj"> <span id="vlink_1_s1"><em onclick="Order(1,'vlink_1',1)">倒序↓</em></span><span id="vlink_1_s2"><em onclick="Order(0,'vlink_1',1)" class="over">顺序↑</em></span> </p>
</div>
<div id="vlink_1" class="videourl clearfix">
<ul>
<li><a title='1' href='/' target="_self">1</a></li>
<li><a title='2' href='/' target="_self">2</a></li>
</ul>
</div>
</div>
</div>
</div>

《所写的部分JS如下》
<script>
function settab(name,name2,cursel,n){
for(i=1;i<=n;i++){
var menu=document.getElementById(name+i);
var con=document.getElementById(name2+i);
menu.className=i==cursel?"on":"";
con.style.display=i==cursel?"block":"none";
}
};


function $$(id){return document.getElementById(id);}
function Order(o,id,vi){
var tag,leng,i,phtml,box,ubox,uhtml,isno,s1,s2
box=$$(id);
tag=box.getElementsByTagName('li');
leng=tag.length;
uhtml="";
if (o==1){
for(i=leng-1;i>=0;i--){
if(i==leng-1){isno='<li class="new">';}else{isno='<li>';}
uhtml+=isno+tag[i].innerHTML+"</li>";
}
s1="<em class=\"over\">倒序↓</em>"
s2="<em onclick=\"Order(0,'vlink_"+vi+"',"+vi+")\">顺序↑</em>"
}else{
for(i=leng-1;i>=0;i--){
if(i==0){isno='<li class="new">';}else{isno='<li>';}
uhtml+=isno+tag[i].innerHTML+"</li>";
}
s1="<em onclick=\"Order(1,'vlink_"+vi+"',"+vi+")\">倒序↓</em>"
s2="<em class=\"over\">顺序↑</em>"
}
$$(id+"_s1").innerHTML=s1;
$$(id+"_s2").innerHTML=s2;
uhtml="<ul>"+uhtml+"</ul>";
box.innerHTML=uhtml;
}

var path=window.location.pathname;
var substr = path.match(/\d{1,}-(\d{1,})-\d{1,}/);
var playid=parseInt(substr[1])+1;

var a=document.getElementById("vlink_"+playid).getElementsByTagName("a");
for( var i = 0; i < a.length; i++) {
var tmp = a[i].getAttribute('title');
if(tmp == part) {
a[i].style.color = 'red';
break;
}
}

</script>

现在遇到一个问题，不懂怎么写 给 ul 里的 li id是tab82的 默认添加 一个class 同时也默认给 id stab82也加个css: display:block;

新手，查阅了好多资料，写得一团糟，现在求大神 麻烦帮写写 ，谢谢各位大神