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