offsetX / offsetY

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  border: solid 3px steelblue;
}

#parent {
  cursor: crosshair;
  border: solid 7px steelblue;
  position: relative;
  margin: 29px auto;
  padding: 31px;
  width: 800px;
  height: 400px;
  background: #eee;
}

#child {
  position: absolute;
  border: solid 13px steelblue;
  margin: 17px;
  padding: 23px;
  top: 100px;
  left: 100px;
  width: 200px;
  height: 100px;
  background: #ccc;
}

#circle {
  position: absolute;
  pointer-events: none;
  border-radius: 12px;
  margin-left: -12px;
  margin-top: -12px;
  width: 24px;
  height: 24px;
  background: red;
}

#small-circle {
  position: absolute;
  pointer-events: none;
  background: black;
  border-radius: 6px;
  top: 12px;
  left: 12px;
  margin-left: -4px;
  margin-top: -4px;
  width: 8px;
  height: 8px;
}

</style>

d3.select("#child").on("mousemove", function() {
  var ox, oy;

if ("offsetX" in d3.event) {
    ox = d3.event.offsetX - this.clientLeft;
    oy = d3.event.offsetY - this.clientTop;
  } else {
    ox = d3.event.layerX - this.clientLeft;
    oy = d3.event.layerY - this.clientTop;
  }

d3.select("#offset").text(ox + "," + oy);

d3.select("#circle")
      .style("left", ox + "px")
      .style("top", oy + "px");
});

</script>

https://d3js.org/d3.v3.min.js