JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a">#a</div>
<div id="b">#b</div>

CSS

#a, #b {
  height: 50px;
  background-color: #000000;
  color: #ffffff;
  opacity: 0.5;
  transition: opacity 0.5s;
}

#a {
  width: 100%;
  display: table;
}

#a:hover, #b:hover {
  opacity: 1;
}