• index.html

<image class="bg" src="/images/bg.png"></image>

• index.css

 .bg {
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    margin: auto;
    content: "";
    width: 300%;
    height: 300%;
    z-index: -1;
    filter: blur(10px);
}