var divs = document.querySelectorAll('span');
for (var i = 0; i < divs.length; i++) {
  var time = 100 * i;
  anim(divs[i],'width','%',0,100,time);
  anim(divs[i],'opacity',0,0,1,time);
}

// anim(foo,'opacity',0,0,1,1000)
function anim(selector, style, unit, from, to, time) {
    if (!selector) return;
    var start = new Date().getTime(),
        timer = setInterval(function() {
            var step = Math.min(1, (new Date().getTime() - start) / time);
            selector.style[style] = (from + step * (to - from)) + unit;
            if (step == 1) clearInterval(timer);
        }, 25);
    selector.style[style] = from + unit;
};