<div class="box">
  <form autocomplete="off">
    <h2>登录</h2>
    <div class="inputBox">
      <input type="text" required="required">
      <span>用户名</span>
      <i></i>
    </div>
    <div class="inputBox">
      <input type="password" required="required">
      <span>密码</span>
      <i></i>
    </div>
    <div class="links">
      <a href="http://www.webqdkf.com">忘记密码 ？</a>
      <a href="http://www.webqdkf.com">注册</a>
    </div>
    <input type="submit" value="登录">
  </form>
</div>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: 'Poppins', sans-serif;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  flex-direction: column;
  background: #23242a;
}

.box {
  position: relative;
  width: 380px;
  height: 420px;
  background: #1c1c1c;
  border-radius: 8px;
  overflow: hidden;
}

.box::before {
  content: '';
  z-index: 1;
  position: absolute;
  top: -50%;
  left: -50%;
  width: 380px;
  height: 420px;
  transform-origin: bottom right;
  background: linear-gradient(0deg,transparent,#45f3ff,#45f3ff);
  animation: animate 6s linear infinite;
}

.box::after {
  content: '';
  z-index: 1;
  position: absolute;
  top: -50%;
  left: -50%;
  width: 380px;
  height: 420px;
  transform-origin: bottom right;
  background: linear-gradient(0deg,transparent,#45f3ff,#45f3ff);
  animation: animate 6s linear infinite;
  animation-delay: -3s;
}

@keyframes animate {
  0% {
    transform: rotate(0deg);
  }
  100% {
    transform: rotate(360deg);
  }
}

form {
  position: absolute;
  inset: 2px;
  background: #28292d;
  padding: 50px 40px;
  border-radius: 8px;
  z-index: 2;
  display: flex;
  flex-direction: column;
}

h2 {
  color: #45f3ff;
  font-weight: 500;
  text-align: center;
  letter-spacing: 0.1em;
}

.inputBox {
  position: relative;
  width: 300px;
  margin-top: 35px;
}

.inputBox input {
  position: relative;
  width: 100%;
  padding: 20px 10px 10px;
  background: transparent;
  outline: none;
  box-shadow: none;
  border: none;
  color: #23242a;
  font-size: 1em;
  letter-spacing: 0.05em;
  transition: 0.5s;
  z-index: 10;
}

.inputBox span {
  position: absolute;
  left: 0;
  padding: 20px 0px 10px;
  pointer-events: none;
  font-size: 1em;
  color: #8f8f8f;
  letter-spacing: 0.05em;
  transition: 0.5s;
}

.inputBox input:valid ~ span,
.inputBox input:focus ~ span {
  color: #45f3ff;
  transform: translateX(0px) translateY(-34px);
  font-size: 0.75em;
}

.inputBox i {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 2px;
  background: #45f3ff;
  border-radius: 4px;
  overflow: hidden;
  transition: 0.5s;
  pointer-events: none;
  z-index: 9;
}

.inputBox input:valid ~ i,
.inputBox input:focus ~ i {
  height: 44px;
}

.links {
  display: flex;
  justify-content: space-between;
}

.links a {
  margin: 10px 0;
  font-size: 0.75em;
  color: #8f8f8f;
  text-decoration: beige;
}

.links a:hover, 
.links a:nth-child(2) {
  color: #45f3ff;
}

input[type="submit"] {
  border: none;
  outline: none;
  padding: 11px 25px;
  background: #45f3ff;
  cursor: pointer;
  border-radius: 4px;
  font-weight: 600;
  width: 100px;
  margin-top: 10px;
}

input[type="submit"]:active {
  opacity: 0.8;
}