<!DOCTYPE html>
<meta name="viewport" content="width=device-width,initial-scale=1">
<style>
html, body {
  margin: 0;
  width: 100%;
  height: 100%;
}
div {
  position: absolute;
}
</style>
<div style="background: green; width: 100%; height: 100%;"></div>
<div style="background: blue; width: 50%; height: 50%;"></div>