​x
 
<html>
<head>
<style type="text/css">
p
{ 
border: thick solid #FF0000
}
</style>
<script type="text/javascript">
function changeBorderColor()
{
document.getElementById("p1").style.borderTopColor="#0000FF";
}
</script>
</head>
<body>
​
<input type="button" onclick="changeBorderColor()" value="Change top border color" />
​
<p id="p1">This is a paragraph</p>
​
</body>
</html>
​