JSFiddle - React, Tailwind, and code Playground

HTML

<div class="div">
<button class="button">Hello</button>
</div>
<footer class="footer">This is footer</footer>

CSS

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

.div {
  display: flex;
  justify-content: center;
  align-content: center;
  height: 100%;
  /* position: fixed;
  bottom: 0;
  right: 0;
  left: 0;
  top: 0; */
  background-color: green;
}

.button {
  transition: 1s;
  opacity: 1;
}

.button:hover {
  /* background-color: transparent; */
  opacity: 0.5;
}

.footer {
  position: fixed;
  bottom: 0;
  height: 50px;
  background-color: red;
  width: 100%;
  text-align: center;
  color: white;
}