基于HTML、CSS与JavaScript的登录注册界面实现

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>欢迎你，请先登陆！</title>
 <script type="text/javascript" src="../static/jsp/lx.js"></script>
 <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet">
 <link rel="stylesheet" href="../static/css/lx.css">

</head>
<body>
<div class="box">
 <h2>登陆</h2>

 <div class="input_box">
  <input id="uname" type="text" name="user" placeholder="请输入用户名">
 </div>
 <div class="input_box">
  <input id="upass" type="password" name="psw" placeholder="请输入密码">
 </div>
 <div id="error_box"><br></div>
 <div class="input_box">
  <button type="button" class="btn btn-primary" onclick="fnLogin()">登陆</button>&nbsp&nbsp&nbsp&nbsp
  <a href="regist.html"><input type="button" class="btn btn-info" name="regist" value="注册"></a>
 </div>



</div>
</body>
</html>

*{
 margin: 0;
 padding: 0;
 font-family: 微软雅黑;
 font-size: 12px;
}
.box{
 width: 390px;
 height: 320px;
 border: solid 1px #ddd;
 background: #FFF;
 position: absolute;
 left: 50%;
 top:42%;
 margin-left: -195px;
 margin-top: -160px;
 text-align: center;
}
.box h2{
 font-weight: normal;
 color:#666;
 font-size: 16px;
 line-height: 40px;
 overflow: hidden;
 text-align: center;
 border-bottom: solid 1px #ddd;
 background: #f7f7f7;
}
.input_box{
 width:350px;
 padding-bottom: 15px;
 margin: 0 auto;
 overflow: hidden;
}

function fnLogin() {
 var oUname = document.getElementById("uname")
 var oUpass = document.getElementById("upass")
 var oError = document.getElementById("error_box")
 var isError = true;
 if (oUname.value.length > 20 || oUname.value.length < 6) {
  oError.innerHTML = "用户名请输入6-20位字符";
  isError = false;
  return;
 }else if((oUname.value.charCodeAt(0)>=48) && (oUname.value.charCodeAt(0)<=57)){
  oError.innerHTML = "首字符必须为字母";
  return;
 }else for(var i=0;i<oUname.value.charCodeAt(i);i++){
  if((oUname.value.charCodeAt(i)<48)||(oUname.value.charCodeAt(i)>57) && (oUname.value.charCodeAt(i)<97)||(oUname.value.charCodeAt(i)>122)){
   oError.innerHTML = "必须为字母跟数字组成";
   return;
  }
 }

 if (oUpass.value.length > 20 || oUpass.value.length < 6) {
  oError.innerHTML = "密码请输入6-20位字符"
  isError = false;
  return;
 }
 window.alert("登录成功")
}

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>欢迎你，请先登陆！</title>
 <script type="text/javascript" src="../static/jsp/lx.js"></script>
 <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet">
</head>
<body>
<div id="container" style="width: 400px" align="center">
 <div id="header" style="background-color: aquamarine">
  <h2 align="center">注册</h2>
 </div>
 <div id="content">
  <p align="center">账号:
   <input id="uname" type="text" name="user" placeholder="账号首字符为字母">
  </p>
  <p align="center">密码:
   <input id="upass" type="password" name="psw" placeholder="请输入密码">
  </p>
  <div id="error_box"><br></div>
  <br>

  <button onclick="fnLogin()" class="btn btn-info">注册</button>
 </div>
 <div style="background-color: aquamarine">
  <i>版权信息@</i>
 </div>
</div>
</body>
</html>