document.ElementById("id").style.样式=“”；

注意：document.getElementByClassName（“class”）   返回这个class数组  [0]

例子：    

document.getElementById("del-ad").οnclick=function(){

    document.getElementsByClassName("advert")[0].style.opacity="0";

    document.getElementsByClassName("advert")[0].style.transition="1s";

    setTimeout(

        function(){

            document.getElementsByClassName("advert")[0].style.display="none";

        },1000

    );

}