<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<p ondblclick="alert(123)">ppppp</p>
<input class="keyword" type="text" onfocus="func1()" onblur="func2()" value="请输入用户名">
<input type="text">
<script>
function func1() {
// console.log(111)
var ky=document.getElementsByClassName("keyword")[0];
ky.value="";
}
function func2() {
var ky=document.getElementsByClassName("keyword")[0];
if (ky.value.trim().length==0){
ky.value="请输入用户名";
}
}
</script>
</body>
</html>
原文：https://www.cnblogs.com/gerenboke/p/11767313.html