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chromium/content/test/data/accessibility/event/checkbox-validity.html 

<!DOCTYPE html>
<form>
    <input id="in1" type="checkbox" required>
    <input id="in2" type="checkbox" required checked="true">
</form>
<script>
    function go() {
        document.getElementById('in1').checked = true;
        document.getElementById('in2').checked = false;
    }
</script>