sousa-gecko/testing/web-platform/tests/css/css-position/containing-block-change-button-ref.html

<!doctype html>
<meta charset="utf-8">
<style>
  #button {
    width: 400px;
    height: 400px;
    margin: 50px;
    padding: 0;
    border: 0;
    background-color: purple;
    vertical-align: top;
  }
  #button > div {
    width: 100px;
    height: 100px;
    background-color: green;
  }
</style>
<button id="button">
  <div></div>
</button>