<!doctype html>
<title>Test reference</title>
<style>
  body { margin: 0 }

  #outer, #inner {
    background-color: purple;
    width: 50%;
    height: 100vh;
  }

  #inner {
    position: absolute;
    background-color: blue;
    top: 0;
    left: 50%;
  }
</style>
<div id="outer">
  <div id="inner"></div>
</div>