<!doctype html>
<title>CSS test reference</title>
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<style>
  .ancestor {
    width: 100px;
    height: 100px;
    position: fixed;
    left: 100px;
    top: 0;
    background-color: green;
  }
</style>
<div class="ancestor"></div>