​x
 
<!DOCTYPE html>
<html>
<head>
<style>
#myDiv {
  border: 1px solid #FF0000;
  margin: 2cm 4cm 3cm 2cm;
}
</style>
</head>
<body>
​
<div id="myDiv">这是 div。</div>
<br>
<button type="button" onclick="myFunction()">改变外边距</button>
​
<script>
function myFunction() {
  document.getElementById("myDiv").style.margin = "25px";
}
</script>
​
</body>
</html>
​