<!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>
  html, body { margin: 0 }
  #box {
    width: 100px;
    height: 100px;
    background: green;
  }
</style>
<div id="box"></div>