​x
 
<!DOCTYPE html>
<html>
<head>
<style>
.mystyle {
  margin: 20px;
  width: 300px;
  height: 50px;
  background-color: coral;
  color: white;
  font-size: 25px;
}
</style>
</head>
<body>
​
<p>单击按钮可将“mystyle”类添加到 DIV。</p>
​
<button onclick="myFunction()">试一试</button>
​
<div id="myDIV">
I am a DIV element
</div>
<br>
​
<script>
function myFunction() {
  document.getElementById("myDIV").classList.add("mystyle");
}
</script>
​
</body>
</html>
​