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chromium/third_party/blink/web_tests/external/wpt/css/mediaqueries/prefers-color-scheme.html 

<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/mediaqueries-5/#prefers-color-scheme" />
<script type="text/javascript" src="/resources/testharness.js"></script>
<script type="text/javascript" src="/resources/testharnessreport.js"></script>

<script type="text/javascript" src="resources/matchmedia-utils.js"></script>
<script>
query_should_be_known("(prefers-color-scheme)");
query_should_be_known("(prefers-color-scheme: light)");
query_should_be_known("(prefers-color-scheme: dark)");

query_should_be_unknown("(prefers-color-scheme: 0)");
query_should_be_unknown("(prefers-color-scheme: none)");
query_should_be_unknown("(prefers-color-scheme: 10px)");
query_should_be_unknown("(prefers-color-scheme: dark 0)");
query_should_be_unknown("(prefers-color-scheme: dark light)");
query_should_be_unknown("(prefers-color-scheme: light/dark)");
query_should_be_unknown("(prefers-color-scheme: no-preference)");

test(() => {
  let booleanContext = window.matchMedia("(prefers-color-scheme)");
  assert_true(booleanContext.matches);
}, "Check that prefer-color-scheme evaluates to true in the boolean context");
</script>