<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
*{padding: 0;margin: 0;}
#search{
position: relative;
}
input{
outline: none;
display: block;
width: 490px;
height: 50px;
margin-top: 20px;
font-size: 20px;
border: 2px solid orange;
border-radius: 10px;
}
label{
position: absolute;
top: 20px;
left: 10px;
font-size:8px;
color: gray;
}
</style>
</head>
<body>
<div id="search">
<input type="text" id="text" />
<label for="txt" id="msg">路飞学城</label>
</div>
</body>
<script type="text/javascript">
var txt = document.getElementById(‘text‘);
var msg = document.getElementById(‘msg‘);
//检测用户表单输入的时候
txt.oninput = function(){
if (this.value == ‘‘) {
msg.style.display = ‘block‘
}else{
msg.style.display = ‘none‘
}
}
</script>
</html>
原文：https://www.cnblogs.com/beallaliu/p/9348032.html