NastyaNewer,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<div id="box"></div>
<style type="text/css">
#box{
border: 1px dashed Gray; padding: 5px; height: 100px; width: 100px;
background-color: #0000CD;
opacity: 0.2;
}
</style>
<script>
var timer;
box.addEventListener('mouseup', function() {
window.clearTimeout(timer);
var op = 20;
function anim(duration) {
box.style.opacity = ++op/100;
if(op < 100) timer = setTimeout(anim, duration, duration);
}
anim(25)
});
</script>
</body>
</html>