<style>
#foo{
position:fixed;
left:50%;
margin-left:-100px;
top:12%;
height:100px;
width:200px;
background-color:#F8F8F8;
border:outset 2px;
transition-property:top;
-webkit-transition-property:top;
-moz-transition-property:top;
-o-transition-property:top;
-ms-transition-property:top;
transition-duration: 0.96s;
-webkit-transition-duration: 0.96s;
-moz-transition-duration: 0.96s;
-o-transition-duration: 0.96s;
-ms-transition-duration: 0.96s; /* IE9+ */
}
</style>
<div id=foo>блаблаблабла блаблаблаблаблаблаблабла я див</div>
<script type="text/javascript">
document.write('<style>#foo{top:45%}</style>')
</script>