CSS效果：简单的登录框

HTML：

<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link rel="stylesheet" href="style.css">
    <title></title>
</head>
<body>
    <form class="box" action="index.html">
        <h1>LOGIN</h1>
        <input class="username" type="text" placeholder="username">
        <input class="userpassword" type="text" placeholder="password">
        <button class="submit" type="submit">login</button>
    </form>
</body>
</html>

　　CSS：

body{
    margin:0;
    padding:0;
    background:#576574;
}
.box{
    position: absolute;
    top:50%;
    left:50%;
    transform: translate(-50%,-50%);
    width:250px;
    height:300px;
    background:#222f3e;
    text-align: center;
}
.box h1{
    color:whitesmoke;
    font-size:30px;
    margin-bottom:36px;
}
.box input{
    margin:25px auto;
    display: block;
    width:160px;
    height:35px;
    border:2px solid #54a0ff;
    border-radius: 30px;
    outline: none;
    background:#222f3e;
    text-align: center;
    color:whitesmoke;
    transition: 0.3s;
}
.submit{
    width:100px;
    height:35px;
    color:white;
    background:#222f3e;
    border:2px solid #54a0ff;
    border-radius: 25px;
    outline: none;
    transition: 0.4s;
    cursor: pointer;
}
.box input:focus{
    width:200px;
    border:2px solid #10ac84;
    transition: 0.3s;
}
.submit:hover{
    border:2px solid #10ac84;
    width:200px;
    transition: 0.4s;
}