[hoax]

как-то так

function animate(opts) {

  var start = new Date;
  var delta = opts.delta || linear;

  var timer = setInterval(function() {
    var progress = (new Date - start) / opts.duration;

    if (progress > 1) progress = 1;

    opts.step( delta(progress) );

    if (progress == 1) {
      clearInterval(timer);
      opts.complete && opts.complete();
    }
  }, opts.delay || 13);

  return timer;
}





function animateProp(opts) {
  var start = opts.start, end = opts.end, prop = opts.prop;

  opts.step = function(delta) {
    var value = Math.round(start + (end - start)*delta);
    opts.elem.style[prop] = value + 'px';
  }
  return animate(opts);
}



















var div = document.createElement('div');

animateProp({ elem: img, prop: 'opacity', start:0.5, end: 1.0, duration: 1000 });