mozilla-release/testing/web-platform/tests/fullscreen/rendering/fullscreen-pseudo-class-manual.html

<!DOCTYPE html>
<title>:fullscreen pseudo-class</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="../trusted-click.js"></script>
<div><div></div></div>
<script>
async_test(t => {
  const outer = document.querySelector("div");
  const inner = outer.firstChild;

  // First request fullscreen for the outer element.
  trusted_request(t, outer);
  document.onfullscreenchange = t.step_func(() => {
    assert_equals(document.fullscreenElement, outer);
    assert_true(outer.matches(':fullscreen'),
                'outer:fullscreen in simple fullscreen');
    assert_false(inner.matches(':fullscreen'),
                 'inner:fullscreen in simple fullscreen');

    // Then request fullscreen for the inner element.
    trusted_request(t, inner);
    document.onfullscreenchange = t.step_func_done(() => {
      // Although inner is the fullscreen element, both elements match the
      // selector, as both have their fullscreen flag set.
      assert_equals(document.fullscreenElement, inner);
      assert_true(inner.matches(':fullscreen'),
                  'inner:fullscreen in nested fullscreen');
      assert_true(outer.matches(':fullscreen'),
                  'outer:fullscreen in nested fullscreen');
    });
  });
});
</script>