<!DOCTYPE html>

  <title>CSS Reftest Reference</title>

  <link rel="author" title="Gérard Talbot" href="http://www.gtalbot.org/BrowserBugsSection/css21testsuite/">

  <style>
  div#black-walls
    {
      border: solid 40px;
      border-color: transparent black;
      height: 400px;
      width: 400px;
    }

  div#gradient
    {
      background-image: linear-gradient(orange, blue);
      height: 100%; /* computes to 400px */
      margin: 0px auto;
      /* the gradient block is horizontally centered inside div#black-walls */
      width: 200px;
    }
  </style>

  <div id="black-walls"><div id="gradient"></div></div>