如何为输入占位符创建退回效果？

<input type="text" placeholder="请输入内容">

input[type="text"] {
  font-size: 18px;
  padding: 10px;
  border: none;
  border-bottom: 1px solid #ccc;
  transition: border-bottom-color 0.3s;
}

input[type="text"]:focus {
  outline: none;
  border-bottom-color: #007bff;
}

input[type="text"]:focus::placeholder {
  animation: moveUp 0.3s forwards;
}

@keyframes moveUp {
  from {
    transform: translateY(0);
    opacity: 1;
  }
  to {
    transform: translateY(-20px);
    opacity: 0;
  }
}