<style type="text/css">
#block {
border:red solid 2px;
height:100px;
width:200px;
text-align:center;
}
</style>
<head>
<body style="height:300%;padding:100px;">
<div id="block">#block</div>
<!-- Весь код - Ниже -->
<script type="text/javascript">
window.onscroll = function(){
var html = document.documentElement, body = document.body;
var BlkStyle = document.getElementById('block').style;
if(html.scrollTop>49||body.scrollTop>49) { //alert()
BlkStyle.borderColor="blue";
BlkStyle.height="34px";
} else BlkStyle.height=BlkStyle.borderColor="";
}
</script>