chromium/chrome/test/data/enterprise/watermark/watermark_test_page.html

<html>
  <style>
    body {
      /* Makes the bottom half of the page black to validate the watermark
         properly renders over dark and light background colors */
      background: linear-gradient(0deg, #000000 50%, #FFFFFF 50%);
    }
  </style>
  <title>Watermark Test Page</title>
  <body>
    <p>
      A page to validate watermarking is correctly overlapping the browser.
    </p>
    <p>
      The text in these paragraphs should be readable despite being overlapped
      by a watermark.
    </p>
  </body>
</html>