chromium/chrome/test/data/forced-colors.html

<!DOCTYPE html>
<title>FAIL</title>
<script>
  if (window.matchMedia("(forced-colors: none)").matches) {
    document.title = "none";
  } else if (window.matchMedia("(forced-colors: active)").matches) {
    document.title = "active";
  }
</script>