登录界面01

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<!DOCTYPE html>
<html lang="zh-CN">
    <head>
        <meta charset="utf-8" />
        <meta name="viewport" content="width=device-width,initial-scale=1.0" />
        <title>用户登录</title>
        <style type="text/css">
                *{
                    /*初始化 清除页面元素的内外边距*/
                    padding: 0;
                    margin: 0;
                    /*盒子模型*/
                    box-sizing: border-box;
                }
                body {
                    /*弹性布局 让页面元素垂直+水平居中*/
                    display: flex;
                    justify-content: center;
                    align-items: center;
                    /*让页面始终占浏览器可视区域总高度*/
                    height: 100vh;
                    /*背景渐变色*/
                    background: linear-gradient(#141e30,#243b55);
 
                }
                .login{
                    /*弹性布局 让子元素称为弹性项目*/
                    display: flex;
                    /*让弹性项目垂直排列  原理是改变弹性盒子的主轴方向  
                    父元素就是弹性盒子  现在改变后的主轴方向是向下了*/
                    flex-direction: column;
                    /*让弹性项目在交叉轴方向水平居中  现在主轴的方向是向下  
                    交叉轴的方向是与主轴垂直 交叉轴的方向是向右*/
                    align-items: center;
                    width: 400px;
                    padding: 40px;
                    background-color: rgba(0, 0, 0, 0.2);
                    box-shadow: 0 15px 25px rgba(0, 0, 0, 0.4);
 
                }
 
                .login h2{
                    color: #fff;
                    margin-bottom: 30px;
                }
                .login .login_box {
                    /*相对定位*/
                    position: relative;
                    width: 100%;
                }
                .login .login_box input{
                    /*清除input框自带的边框和轮廓*/
                    outline: none;
                    border: none;
                    width: 100%;
                    padding: 10px 0;
                    margin-bottom: 30px;
                    color: #fff;
                    font-size: 16px;
                    border-bottom: 1px solid #fff;
                    /*背景颜色为透明色*/
                    background-color: transparent;
                }
 
                .login .login_box label{
                    position:absolute;
                    top: 0 ;
                    left: 0;
                    padding: 10px 0;
                    color: #fff;
                    /*这个属性的默认值是auto 默认是这个元素可以被点击
                    但是如果我们写了none  就是这个元素不能被点击，就好像它可见但是不能用  
                    可望而不可及*/
                    /*这个就是两者的区别*/
                    pointer-events: none;
                    /*加个过度*/
                    transition: all 0.5s;
                }
                /*: focus 选择器是当input获得焦点是触发的样式 + 是相邻兄弟选择器
                    去找与input相邻的兄弟label*/
                /*：valid 选择器是判断input 框的内容是否合法，如果合法会执行下面的属性代码，
                    不合法就不会执行，我们刚开始写布局的时候给input框写了required 我们删掉看对比 
                    当没有required的话   input框的值就会被认为一直合法，所以一直都是下方的样式，
                    但是密码不会，密码框的值为空，那么这句话就不合法，required不能为空
                    当我们给密码框写点东西的时候才会执行以下代码
 
                */
                .login .login_box input:focus + label,
                .login  .login_box input:valid + label{
                    top: -20px;
                    color: #03e9f4;
                    font-size: 12px;
                }
 
                .login a{
                    /*overflow: hidden;*/
                    position: relative;
                    padding: 10px 20px;
                    color: #03e9f4;
                    /*取消a表现原有的下划线*/
                    text-decoration: none;
                    /*同样加个过渡*/
                    transition: all 0.5s;
                }
                .login a:hover {
                    color: #fff;
                    border-radius: 5px;
                    background-color: #03e9f4;
                    box-shadow: 0 0 5px #03e9f4,0 0 25px #03e9f4,0 0 50px #03e9f4,0 0 100px #03e9f4;
                }
                .login a span{
                    position: absolute;
                }
                .login a span:first-child {
                    top: 0;
                    left: -100%;
                    width: 100%;
                    height: 2px;
                    /*to right 就是往右边 下面的同理*/
                    background: linear-gradient(to right,transparent,#03e9f4);
                    /*动画 名称  时长 linear是匀速运动 infinite是无限次运动*/
                    animation: move1 1s linear infinite;
 
                }
                .login a span:nth-child(2){
                    right: 0;
                    top: -100%;
                    width: 2px;
                    height: 100%;
                    background: linear-gradient(transparent,#03e6f4);
                    /*这里多了个0.25s其实是延迟时间*/
                    animation: move2 1s linear 0.25s infinite;
                }
 
                .login a span:nth-child(3){
                    right: -100%;
                    bottom: 0;
                    width: 100%;
                    height: 2px;
                    background: linear-gradient(to left,transparent,#03e9f4);
 
                    animation: move3 1s linear 0.5s infinite;
                }
 
                .login a span:last-child{
                    left: 0;
                    bottom: -100%;
                    width: 2px;
                    height: 100%;
                    background: linear-gradient(#03e9f4,transparent);
                    animation: move4 1s linear 0.75s infinite;
                }
                /*写一下动画 */
                @keyframes move1{
                    0%{
                        left: -100%;
 
                    }
                    50%,
                    100%{
                        left: 100%;
                    }
                }
 
                @keyframes move2{
                    0%{
                        top: -100%;
 
                    }
                    50%,
                    100%{
                        top: 100%;
                    }
                }
 
                @keyframes move3{
                    0%{
                        right: -100%;
 
                    }
                    50%,
                    100%{
                        right: 100%;
                    }
                }
 
                @keyframes move4{
                    0%{
                        bottom: -100%;
 
                    }
                    50%,
                    100%{
                        bottom: 100%;
                    }
                }
        </style>
    </head>
 
    <body>
         <div class="login">
            <h2>用户登录</h2>
            <div class="login_box">
                <!-- required就是不能为空  必须在css效果中有很大的作用 -->
                <input type="text" name='name' id='name' required  />
                <label for="name" >用户名</label>
            </div>
            <div class="login_box">
                <input type="password" name='pwd' id='pwd' required="required">
                <label for="pwd">密码</label>
            </div>
            <a href="javascript:void(0)">
                登录
                <span></span>
                <span></span>
                <span></span>
                <span></span>
            </a>
 
         </div>
 
    </body>
</html>