​x
 
<html>
<head>
<script type="text/javascript" src="/jquery/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
  $(".btn1").click(function(){
    $("p").height("50%");
  });
  $(".btn2").click(function(){
    $("p").height("20em");
  });
});
</script>
</head>
<body>
<p style="background-color:yellow">This is a paragraph.</p>
<button class="btn1">把高度设置为 50%</button>
<button class="btn2">把高度设置为 20 em</button>
</body>
</html>
​