quirks/body-fills-html-quirk-inline.html

<!DOCTYPE quirks-mode>
<link rel="help" href="https://quirks.spec.whatwg.org/#the-body-element-fills-the-html-element-quirk">
<link rel="match" href="body-fills-html-quirk-ref.html">
<style>
body {
  border: solid;
  display: inline-block;
}
span {
  display: inline-block;
  width: 100px;
  height: 100px;
  background: green;
}
</style>
<body>
  <span></span>
</body>