offsetTop.js

function getOffset(el) {
    if (typeof el !== 'undefined') {
        if (el.getBoundingClientRect()) {
            var box = el.getBoundingClientRect(),
                body = document.body,
                docElem = document.documentElement,
                scrollTop = window.pageYOffset || docElem.scrollTop || body.scrollTop,
                clientTop = docElem.clientTop || body.clientTop || 0,
                top = box.top + scrollTop - clientTop;
            return Math.round(top);
        } else {
            var top = 0,
                left = 0
            while (el) {
                top = top + parseInt(el.offsetTop)
                el = el.offsetParent
            }
            return top
        }
    }
}

Example: https://jsfiddle.net/8ofvw7rx/66/