Пробуем так
<html><head><title></title></head>
<body>
<script>
window.onload=function(){
obj = document.getElementById("li_1");
obj.style.opacity = 0;
while (obj.style.opacity < 1 ) {
alert("opacity = "+obj.style.opacity);
obj.style.opacity = +obj.style.opacity + 0.2;
}
}
</script>
<ul class="slider" id="idSlider">
<li id="li_1"><div style="width:100px;height:100px;background-color:red;"></div></li>
</ul>
</body>
</html>