为什么绝对定位的元素使用blur()滤镜无效？

Question

<div class="wrapper" style="position: relative; width: 100vw; height: 100vh; background-color: red;">
    <div class="aa"></div>
    啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊
</div>

.aa{
    background:hsla(0,0%,100%,.25) border-box;    
    width: 100vw;
    height: 100vh;
    position: absolute;
    top: 0;
    left: 0;
    z-index: 100;
    -webkit-filter:blur(20px);
    filter:blur(20px);
}