为啥我加上setIntval()之后，就直接拉不下来了呢？
window.onload = function(){
var obtn = document.getElementById('btn');
var timer = null;
obtn.onclick = function(){
timer = setInterval(function(){
var ostop = document.documentElement.scrollTop || document.body.scrollTop;
document.documentElement.scrollTop = document.body.scrollTop -=200;
},30);
}
}
