<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.bg{
background-color: yellow;
}
.aa{
display:block;
}
.ab{
display:block;
}
span{
display:none;float:left;
}
input{float:left;}
</style>
</head>
<body>
<input type="text" id="name" >
<span id="yhm">请输入3至6位数的用户名</span>
<br><br>
<input type="text" id="pw" >
<span id="mm"> 请输入6至8位数的密码</span>
<br><br>
<input type="button" id="btn" value="提交">
<script>
var btn = document.getElementById("btn");
var username = document.getElementById('name');
var pw = document.getElementById('pw');
var yhm = document.getElementById("yhm");
var mm = document.getElementById("mm");
btn.onclick = function(){
if(username.value.length < 3 || username.value.length > 6){
username.className = "bg";
yhm.className = "aa";
}else{
username.className = "";
yhm.className = "";
}
if(pw.value.length < 6 || pw.value.length > 8){
pw.className = "bg";
mm.className = "ab";
}else{
pw.className = "";
mm.className = "";
}
};
</script>
</body>
</html>
