<!DOCTYPE html>
<html>
<head>
<style>
#myDiv {
border-style: solid;
}
</style>
</head>
<body>
<div id="myDiv">This is a div element.</div>
<br>
<button type="button" onclick="myFunction()">Change width of the bottom border</button>
<script>
function myFunction() {
document.getElementById("myDiv").style.borderBottomWidth = "thin";
}
</script>
</body>
</html>