offsetWidth表示對物件的可見寬度

offsetWidth=width+padding+border

而offsetLeft是盒子邊緣到上一級盒子內邊框的距離，所以跟margin值有關，與border和padding沒有關係。 

offsetLeft=left+margin

*{margin: 0;padding: 0;}
			#div1{
				width: 400px;
				height: 400px;
				background-color: #ccc;
				border: 2px solid #999;
				padding:11px;
				margin: 22px;
				
			}
			#div2{
				width: 100px;
				height: 100px;
				background-color: red;
				border: 1px solid #999;
				padding:10px;
				margin: 19px;
			}
		</style>
		<script type="text/javascript">
		window.onload=function(){
			var aDiv=document.getElementById('div1');
			var aDiv2=document.getElementById('div2')
                        alert(aDiv2.offsetWidth)   //122=100+10+10+1+1=width+padding+border
                        alert(aDiv2.offsetHeight)  //122=100+10+10+1+1=height+padding+border
                        alert(aDiv2.offsetLeft)  //54 = 2+11+22+19= div1的border+div1的padding+div1的margin+div2的margin
                        alert(aDiv2.offsetTop)  //54
		}
		</script>