<!doctype html>
<title>CSS Test Reference</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<style>
  #outer {
    width: 100px;
    height: 100px;
  }
  #inner {
    width: 100%;
    height: 100%;
    background: green;
    border-radius: 99px;
  }
</style>
<p>Should be a green circle below</p>
<div id="outer">
  <div id="inner"></div>
</div>