body {
    overflow: hidden;
    background-color: black;
}
canvas {
    position: absolute;
    top: calc( 50% - 300px );
    left: calc( 50% - 300px );
}
#overlay {
    width: 600px;
    height: 600px;
    position: absolute;
    top: calc( 50% - 300px );
    left: calc( 50% - 300px );
    background: radial-gradient( 300px at 300px 300px, rgba(0,0,0,0) 0%, rgba(0,0,0,1) 100% );
}