cs2.html

<div id='colorsquare2'></div>

<script>
  var cs2 = window.colorsquare2
  cs2.style.cssText = `
    width: 200px;
    height: 200px;
    border: 2px solid black;
  `
  window.addEventListener('mousemove', function (evt) {
    var x = evt.clientX / window.innerWidth
    var y = evt.clientY / window.innerHeight
    var hue = Math.round(y * 360)
    var light = Math.round(x * 100)
    var color = `hsl(${hue}, 100%, ${light}%)`
    cs2.style.background = color
  })
</script>