* {
    padding: 0;
    margin: 0;
}

body {
    width: 100%;
    height: 100vh;
    /* 使用 flex 使得 loginBox 在中间 */
    display: flex;
    justify-content: center;
    align-items: center;
    position: relative;
    /*background-color: rgb(245, 255, 250);*/
}

.bg {
    width: 100%;
    height: 100%;
    position: absolute;
    left:50%;
    top:50%;
    transform: translate(-50%, -50%);

}

.loginBox {
    width: 500px;
    height: 364px;
    color: #03e9f4;
    border-radius: 10px;
    background-color: rgba(0, 0, 0, .8);
    box-shadow: 0px 15px 25px 0px rgba(0, 0, 0, .6);
    padding: 40px;
    box-sizing: border-box;
    z-index: 888;
}

h2 {
    text-align: center;
    color: #fff;
    margin-bottom: 30px;
}

.item {
    height: 45px;
    border-bottom: 1px solid #fff;
    margin-bottom: 40px;
    position: relative;
}

.item input {
    width: 100%;
    height: 100%;
    color: #fff;
    /* 输入框输入光标为白色 */
    padding-top: 20px;
    box-sizing: border-box;
}


/* valid 合法状态下  focus 获取焦点时*/

.item input:focus + label,
.item input:valid + label {
    top: 0;
    font-size: 12px;
}

.item label {
    position: absolute;
    left: 0;
    top: 12px;
    transition: all .2s linear;
    /* linear 匀速 */
}


/* 重置输入框和按钮的样式 */

input,
button {
    background: transparent;
    border: none;
    outline: none;
}

.btn {
    padding: 10px 20px;
    margin-top: 30px;
    color: #03e9f4;
    position: relative;
    overflow: hidden;
}

.btn:hover {
    background-color: #03e9f4;
    border-radius: 5px;
    color: #fff;
    /* text-transform: uppercase; */
    /* 文字大写 */
    /* letter-spacing: 2px; */
    /* 字母间隔 */
}

.btn > span {
    position: absolute;
}

.btn > span:nth-child(1) {
    width: 100%;
    height: 2px;
    background-image: linear-gradient(to right, transparent, #03e9f4);
    left: -100%;
    top: 0;
    animation: span-line-one 1s linear infinite;
}

.btn > span:nth-child(2) {
    width: 2px;
    height: 100%;
    background-image: linear-gradient(to bottom, transparent, #03e9f4);
    right: 0;
    top: -100%;
    /* 延迟 0.25s */
    animation: span-line-two 1s .25s linear infinite;
}

.btn > span:nth-child(3) {
    width: 100%;
    height: 2px;
    background-image: linear-gradient(to left, transparent, #03e9f4);
    left: 100%;
    bottom: 0;
    /* 延迟 0.5s */
    animation: span-line-three 1s .5s linear infinite;
}

.btn > span:nth-child(4) {
    width: 2px;
    height: 100%;
    background-image: linear-gradient(to top, transparent, #03e9f4);
    left: 0;
    bottom: -100%;
    /* 延迟 0.75s */
    animation: span-line-four 1s .75s linear infinite;
}

@keyframes span-line-one {
    50%,
    100% {
        left: 100%;
    }
}

@keyframes span-line-two {
    50%,
    100% {
        top: 100%;
    }
}

@keyframes span-line-three {
    50%,
    100% {
        left: -100%;
    }
}

@keyframes span-line-four {
    50%,
    100% {
        bottom: 100%;
    }
}