JSFiddle - React, Tailwind, and code Playground

HTML

<div class="bt">碰觸我
</div>
<div id="a">
</div>

CSS

#a {
  width: 10em;
  height: 0em;
  background: #333;
}
.bt:hover ~ #a {
  animation: cd-fade-in-1 2s;
  animation-fill-mode: forwards;
}

@keyframes cd-fade-in-1 {
  0% {
    display: block;
    height: 0em;
    visibility: hidden;
  }
  100% {
    display: block;
    height: 10em;
    visibility: visible;
  }
}

/*-------------------*/
.bt{
  width:4em;
  background: blue;
  margin-bottom:2em;
  color:white;
}

JavaScript

/* $(".bt").mouseenter(function() {
  $("#a").removeClass("cd-fade-in")
  $("#a").addClass("cd-fade-in")
}); */