传统的表单控件textarea元素中IE9-浏览器不支持的属性的示例是什么？

<input id="btn1" type="button" value="placeholder='123'">
<input id="btn2" type="button" value="placeholder='abc'">
<input id="btn3" type="button" value="必填">
<input id="btn4" type="button" value="可不填">
<form action="#">
    <textarea id="test" placeholder="测试内容"></textarea>    
    <input type="submit">
</form>
<script>
var test = document.getElementById('test');
btn1.onclick = function(){
    test.setAttribute('placeholder','123');
};
btn2.onclick = function(){
    test.setAttribute('placeholder','abc');
};    
btn3.onclick = function(){
    test.setAttribute('required','');
};
btn4.onclick = function(){
    test.removeAttribute('required');
};
</script>