使table背景颜色交替变换

<!--使table背景颜色交替变换--> 
<script> 
var table1 = document.getElementById("tbname"); 
var rows = table1.getElementsByTagName("tr"); 
for(var i=0;i<rows.length;i++){ 
 if(i%2==0){ 
  rows[i].style.backgroundColor='red'; 
 }else{ 
     rows[i].style.backgroundColor='green'; 
 } 
} 
</script>