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

html {
    height: 100%;
}

body {
    margin: 0;
    padding: 0;
    font-family: sans-serif;
    background-image: linear-gradient(#141e30, #243b55);
}
  
.login-box {
    position: absolute;
    top: 50%;
    left: 50%;
    max-width: 800px;
    width: 400px;
    padding: 40px;
    transform: translate(-50%, -50%);
    background-color: rgba(0, 0, 0, .5);
    box-sizing: border-box;
    box-shadow: 0 15px 25px rgba(0,0,0,.6);
    border-radius: 10px;
}
  
.login-box .login-box__title {
    margin: 0 0 30px;
    color: #fff;
    text-align: center;
}
  
.login-box .user-box {
    position: relative;
}
  
.login-box .user-box input {
    width: 100%;
    padding: 10px 0;
    font-size: 16px;
    color: #fff;
    margin-bottom: 30px;
    border: none;
    border-bottom: 1px solid #fff;
    outline: none;
    background: transparent;
}

.login-box .user-box span {
    position: absolute;
    top:0;
    left: 0;
    font-size: 16px;
    color: #fff;
    pointer-events: none;
    transition: .5s;
}
  
.login-box .user-box input:focus ~ span,
.login-box .user-box input:valid ~ span {
    top: -20px;
    left: 0;
    color: #03e9f4;
    font-size: 12px;
}
  
.login-box form .form-btn {
    position: relative;
    display: inline-block;
    padding: 10px 20px;
    color: #03e9f4;
    background-color: transparent;
    border: none;
    outline: none;
    cursor: pointer;
    font-size: 16px;
    text-transform: uppercase;
    overflow: hidden;
    transition: .5s;
    margin-top: 40px;
    letter-spacing: 4px
}
  
.login-box .form-btn:hover {
    background: #03e9f4;
    color: #fff;
    border-radius: 5px;
    box-shadow: 0 0 5px #03e9f4,
                0 0 25px #03e9f4,
                0 0 50px #03e9f4,
                0 0 100px #03e9f4;
}
  
.login-box .form-btn span {
    position: absolute;
    display: block;
}
  
.login-box .form-btn span:nth-child(1) {
    top: 0;
    left: -100%;
    width: 100%;
    height: 2px;
    background: linear-gradient(90deg, transparent, #03e9f4);
    animation: button-animate-one 1s linear infinite;
}

/* Анимация */
@keyframes button-animate-one {
    0% {
      left: -100%;
    }
    50%,100% {
      left: 100%;
    }
}
  
.login-box .form-btn span:nth-child(2) {
    top: -100%;
    right: 0;
    width: 2px;
    height: 100%;
    background: linear-gradient(180deg, transparent, #03e9f4);
    animation: button-animate-two 1s linear infinite;
    animation-delay: .25s
}
  
@keyframes button-animate-two {
    0% {
      top: -100%;
    }
    50%,100% {
      top: 100%;
    }
}
  
.login-box .form-btn span:nth-child(3) {
    bottom: 0;
    right: -100%;
    width: 100%;
    height: 2px;
    background: linear-gradient(270deg, transparent, #03e9f4);
    animation: button-animate-three 1s linear infinite;
    animation-delay: .5s
}
  
@keyframes button-animate-three {
    0% {
      right: -100%;
    }
    50%,100% {
      right: 100%;
    }
}
  
.login-box .form-btn span:nth-child(4) {
    bottom: -100%;
    left: 0;
    width: 2px;
    height: 100%;
    background-image: linear-gradient(360deg, transparent, #03e9f4);
    animation: button-animate-four 1s linear infinite;
    animation-delay: .75s
}
  
@keyframes button-animate-four {
    0% {
      bottom: -100%;
    }

    50%, 100% {
      bottom: 100%;
    }
}