* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html, body {
    height: 100%;
    width: 100%;
    font-family: sans-serif;
    background: #F3F3F3;
    background-position: center center;
    background-size: cover;
    background-repeat: no-repeat;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
}
body {
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction:column;
}

.register-box input {
    outline: none;
    border: none;
    background: transparent;
}

.register-box {
    width: 100%;
    max-width: 400px;
    padding: 40px 30px;
    background-color: black;
    box-shadow: 0 15px 25px rgba(0, 0, 0, 0.6);
    border-radius: 10px;
    width:100vw !important;
}

    .register-box h2 {
        margin: 0 0 30px;
        color: #fff;
        text-align: center;
    }

    .register-box .user-box {
        position: relative;
        margin-bottom: 30px;
    }

        .register-box .user-box .input {
            width: 100%;
            padding: 10px 0;
            color: #fff;
            border-bottom: 1px solid #fff;
        }

        .register-box .user-box label {
            position: absolute;
            top: 0;
            left: 0;
            padding: 10px 0;
            color: #fff;
            transition: 500ms;
            font-size: 16px;
            pointer-events: none;
        }

    .register-box .input:focus ~ label,
    .register-box .input:valid ~ label {
        top: -20px;
        color: #b9ff66;
        font-size: 12px;
    }

    .register-box button {
        display: flex;
        align-items: center;
        justify-content: center;
        background: #b9ff66;
        padding: 15px 30px;
        color: black;
        margin-top: 20px;
        width: 100%;
        border: none;
        cursor: pointer;
        font-size: 15px;
    }

@media screen and (max-height: 600px) {
    html, body {
        align-items: flex-start;
    }

    .register-box {
        margin-top: 40px;
        margin-bottom: 40px;
    }
}