.btn::after {
    content: "";
    display: block;
    position: absolute;
    width: 100%;
    height: 100%;
    left: var(--x,0); 
    top: var(--y,0);
    pointer-events: none;
    background-image: radial-gradient(circle, #fff 10%, transparent 10.01%);
    background-repeat: no-repeat;
    background-position: 50%;
    transform: translate(-50%,-50%) scale(10);
    opacity: 0;
    transition: transform .3s, opacity .8s;
}
.btn:not([disabled]):active::after {
    transform: translate(-50%,-50%) scale(0);
    opacity: .3;
    transition: 0s;
}