js动画，边框宽度达不到目标值，运行结果总是比目标值小7.

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>多物体运动</title>
<style>
*{list-style:none;margin:0px;padding:0px;}
ul li{width:200px;
	  height:100px;
	  background:yellow;
	  margin:50px auto;
	  border:#000000 solid 1px;
	  filter:alpha(opacity:30);
	  opacity:0.3;
	  }	
</style>
<script>
window.onload=function(){
	var li1=document.getElementById('li1')
	li1.onmouseover=function(){
		startMove(li1,{'width':400,'height':200,opacity:100,borderWidth:50})
	}
	li1.onmouseout=function(){
		startMove(li1,{'width':200,'height':100,opacity:30,borderWidth:1})
	}
	}
//var alpha=30
function startMove(obj,json,fn){
	clearInterval(obj.timer)
	
	obj.timer=setInterval(function(){
		//var icur=0
		var flag=true
		for(var attr in json){
			if(attr=='opacity'){
			var icur=Math.round(parseFloat(getStyle(obj,[attr]))*100)
			}
			else{
			var icur=parseInt(getStyle(obj,[attr]))	
			}
	var speed=(json[attr]-icur)/8
	speed=speed>0?Math.ceil(speed):Math.floor(speed)
			if(icur != json[attr]){
			flag=false;
			}
			if(attr=='opacity'){
			obj.style.filter='alpha(opacity:'+(icur+speed)+')'
			obj.style.opacity=(icur+speed)/100;
			}
			else{
			obj.style[attr]=icur+speed+'px';
			}
		}
		if(flag){
			clearInterval(obj.timer)
			if(fn){fn()}
		}
		
		},50)
		
	}
function getStyle(obj,attr){
	if(obj.currentStyle){
		return obj.currentStyle[attr]
		}
	else{
		return getComputedStyle(obj,false)[attr]
		}
	}
</script>
</head>

<body>
<ul>
	<li id="li1"></li>
</ul>
</body>
</html>

当边框的目标宽度大于8时，运行结果总是比目标宽度小7；

当边框的目标宽度小于等于8时，即使原始宽度不是1，运行结果也总是回变成1；

导致这个bug的原因是什么额，求大神解答？

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