<!doctype html>
<title>CSS Test Reference</title>
<meta charset="utf-8">
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<style>
html,
body {
  height: 100%;
  overflow: hidden;
  margin: 0;
  padding: 0;
  background-color: #222;
}

body {
  display: flex;
  transform: scale(0.9);
}

#fixed {
  position: fixed;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  z-index: 1;
  background: black;
}

#popup {
  position: absolute;
  width: 200px;
  height: 200px;
  left: 0;
  top: 0;
  z-index: 2;
  background: green;
  transform: scale( 1.1111 );
}

body, #popup {
  transform-origin: 50% 50%;
}
</style>
<div id="popup"></div>
<div id="fixed"></div>