Purple dots

If you keep your gaze focused at the black crosshairs, eventually the purple dots fade away and you start seeing an animated bright green dot instead.

{
    var svgMain = d3.select("#wrapper").append("svg")
        .attr("viewBox", "0 0 600 600")
        .attr("width", "100%")
        .attr("height", "100%");

    svgMain.append("defs")
    .append("filter")
    .attr("id", "blur1")
    .attr("x", "-50%")
    .attr("y", "-50%")
    .attr("width", "200%")
    .attr("height", "200%")
    .append("feGaussianBlur")
    .attr("in", "SourceGraphic")
    .attr("stdDeviation", "5");

    svgMain = svgMain
        .append("g")
        .attr("transform", "translate(300, 300)");

    svgMain.append("g")
    .append("path")
    .attr("stroke", "black")
    .attr("fill", "none")
    .attr("d", "M -20 0 L 20 0 M 0 -20 L 0 20");

    let nPoints = 12;
    let angles = d3.range(0, nPoints).map((a) => (a / nPoints) * Math.PI * 2);

    var circles = svgMain.append("g")
        .selectAll("circle")
        .data(angles)
        .enter()
        .append("circle")
        .attr("r", 15)
        .style("filter", "url(#blur1)")
        .attr("cx", (d) => Math.cos(d) * 210)
        .attr("cy", (d) => Math.sin(d) * 210)
        .attr("fill", d3.hcl(0, 70, 50));

    function startLoop(d) {
    d3.select(this)
        .transition()
        .duration(150)
        .style("opacity", 0)
        .transition()
        .duration(150)
        .style("opacity", 1)
        .transition()
        .delay(700)
        .on("end", startLoop);
    }

    circles.transition()
    .delay((d) => (d / (Math.PI * 2)) * 1000)
    .on("end", startLoop);
}

Acknowledgements

I tried to track down the original discoverer of this one but ultimately failed; it seems at this point to be folklore.