<script type="text/javascript" src="/js/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$(".btn1").click(function(){
$("p").html("Hello <b>world!</b>");
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<button class="btn1">改变 p 元素的内容</button>