<meta charset=utf-8>
<title>Upper square green, rest blue</title>
<style>
  div {
    position: absolute;
    width: 50vw;
    height: 50vh;
    top: 0;
    left: 0;
    background-color: lime;
  }
  :root {
    background-color: blue;
  }
</style>
<div></div>
