<div class="inputBox">
  <input type="text" required="required">
  <span>昵称</span>
  <i></i>
</div>
<div class="inputBox">
  <input type="text" required="required">
  <span>姓名</span>
  <i></i>
</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;
  gap: 30px;
  background: #222;
}

.inputBox {
  position: relative;
  width: 250px;
}

.inputBox input {
  width: 100%;
  padding: 10px 0 5px;
  background: transparent;
  outline: none;
  box-shadow: none;
  border: none;
  color: #fff;
  font-size: 1em;
  letter-spacing: 0.1em;
  transition: 0.5s;
}

.inputBox span {
  position: absolute;
  left: 0;
  padding: 10px 0 5px;
  pointer-events: none;
  font-size: 1em;
  color: rgba(255,255,255,0.5);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  transition: 0.5s;
}

.inputBox input:valid ~ span,
.inputBox input:focus ~ span {
  color: #2196f3;
  transform: translateY(-16px);
  font-size: 0.65em;
}

.inputBox i {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 2px;
  background: #fff;
  overflow: hidden;
}

.inputBox i::before {
  content: '';
  position: absolute;
  left: -100%;
  width: 100%;
  height: 100%;
  background: linear-gradient(90deg,#ff1b69,#ff0,#2196f3,#9c27b0,#ff1b69);
  animation: animate 2s linear infinite;
  transition: 0.5s;
}

.inputBox input:valid ~ i::before,
.inputBox input:focus ~ i::before {
  left: 0;
} 

@keyframes animate {
  0% {
    background-position-x: 0;
  }
  
  100% {
    background-position-x: 250px;
  }
}