元素到页面的绝对距离

function getPos(obj){
    var iTop = 0;
    while(obj){
        iTop += obj.offsetTop;
        obj = obj.offsetParent;
    }
    return iTop;
}

span{
    border-top:10px dashed transparent;
    border-left:10px dashed transparent;
    border-right:10px dashed transparent;
    border-bottom:10px solid red;
    width: 0px; height: 0px; display: block;
}