​x
 
<!DOCTYPE html>
<html>
<body>
​
<h2>This is a h2 element</h2>
​
<p>This is a p element</p>
​
<p>This is also a p element.</p>
​
<p>This is also a p element - 单击按钮可更改此文档中所有 p 元素的背景颜色。</p>
​
<button onclick="myFunction()">试一试</button>
​
<script>
function myFunction() {
  var myNodelist = document.getElementsByTagName("P");
  var i;
  for (i = 0; i < myNodelist.length; i++) {
    myNodelist[i].style.backgroundColor = "red";
  }
}
</script>
​
</body>
</html>
​