9-2编程练习中i=1,为什么从1开始
window.onload = function(){
var trs = document.getElementsByTagName('tr');
for(var i = 1; i < trs.length; i++){
trs[i].onmouseover = function(){
this.style.backgroundColor = "#f2f2f2";
}
trs[i].onmouseout = function(){
this.style.backgroundColor = "#fff";
}
}
}