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chromium/chrome/test/data/prefers-color-scheme.html 

<!DOCTYPE html>
<title>FAIL</title>
<script>
  if (window.matchMedia("(prefers-color-scheme: light)").matches) {
    document.title = "light";
  } else if (window.matchMedia("(prefers-color-scheme: dark)").matches) {
    document.title = "dark";
  } else if (window.matchMedia("(prefers-color-scheme: no-preference)").matches) {
    document.title = "no-preference";
  }
</script>