firefox/testing/web-platform/tests/fullscreen/api/document-fullscreen-element.html

<!DOCTYPE html>
<title>Document#fullscreenElement</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<script src="../trusted-click.js"></script>
<div id="log"></div>
<script>
    promise_test(async function (t) {
        const div = document.querySelector("div");

        await trusted_click();

        assert_equals(
            document.fullscreenElement,
            null,
            "fullscreenElement before requestFullscreen()"
        );

        const fsPromise = div.requestFullscreen();
        assert_equals(
            document.fullscreenElement,
            null,
            "fullscreenElement after requestFullscreen()"
        );
        await fsPromise;
        assert_equals(
            document.fullscreenElement,
            div,
            "fullscreenElement before exitFullscreen()"
        );
        const exitPromise = document.exitFullscreen();
        assert_equals(
            document.fullscreenElement,
            div,
            "fullscreenElement after exitFullscreen()"
        );
        await Promise.all([fullScreenChange(), exitPromise]);

        assert_equals(
            document.fullscreenElement,
            null,
            "fullscreenElement after exiting fullscreen"
        );
    });
</script>