​x
 
<!DOCTYPE html>
<html>
<body>
​
<div id="myDiv">这是一个 div 元素。</div>
<br>
<button type="button" onclick="myFunction()">设置下边框</button>
​
<script>
function myFunction() {
  document.getElementById("myDiv").style.borderBottom = "thick solid #0000FF";
}
</script>
​
</body>
</html>
​