html5实现表单的复选框验证

<form id="example1" method="POST" onsubmit="return checkForm(this);">

    <p><input id="field_terms" onchange="this.setCustomValidity(validity.valueMissing ? '请选择接受条款和条件!' : '');" type="checkbox" required name="terms">

    <label for="field_terms">我接受 <u>条款和条件</u></label></p>

    <span><input type="submit"></span>

</form>

<script type="text/javascript">

    document.getElementById("field_terms").setCustomValidity("请选择接受条款和条件!");

</script>