JSFiddle - React, Tailwind, and code Playground

HTML

<div id='gui'>
  <div id='poster'></div>
</div>

CSS

#gui {
  width: 250px;
  height: 50px;
  position: fixed;
  background-color: green;
}

#poster {
  width: 250px;
  height: 250px;
  position: fixed;
  background-color: red;
  z-index: -1;
}