js+css 控制表格隔行变色与单行高亮的代码

<style type="text/css" media="screen"> 
table {border-collapse:collapse;border:solid #999;border-width:1px 0 0 1px;} 
table td {border:solid #999;border-width:0 1px 1px 0;} 
tr.t1 td {background-color:#fff;}/* 第一行的背景色 */
tr.t2 td {background-color:#eee;}/* 第二行的背景色 */
tr.t3 td {background-color:#ccc;}/* 鼠标经过时的背景色 */
tr.t4 td {background-color:#E2EBED;} 
</style>

<table border="1" width="100%" id="tab">
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
<tr><td>11</td><td>11</td><td>11</td></tr>
</table>  

<script type="text/javascript">
var Ptr=document.getElementById("tab").getElementsByTagName("tr"); 
function $() { 
for (i=2;i<Ptr.length+1;i++) {  
Ptr[i-1].className = (i%2>0)?"t1":"t2";  
} 
} 
window.onload=$; 
for(var i=1;i<Ptr.length;i++) { 
Ptr[i].onmouseover=function(){ 
thisthis.tmpClass=this.className; 
this.className = "t3"; 
}; 
Ptr[i].onmouseout=function(){ 
thisthis.className=this.tmpClass; 
}; 
} 
</script>