0055 html5新增表单属性：required、placeholder、autofocus、autocomplete、multiple

    <form action="">
        用户名: <input type="text" required="required" placeholder="请输入用户名" autofocus="autofocus" name="username" autocomplete="off"> <br>
        <input type="submit" value="提交"> <br> 
        上传头像: <input type="file" name="" id="" multiple="multiple"> <br>
    </form>