cs3.html

<div id='colorsquare3'></div>

<script>
  var cs3 = window.colorsquare3
  cs3.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 w = `${x * 200}px`
    var h = `${y * 200}px`
    var trans = `rotate(${x * 360}deg)`
    var color = `hsl(${hue}, 100%, ${light}%)`
    cs3.style.background = color
    cs3.style.width = w
    cs3.style.height = h
    cs3.style.transform = trans
  })
</script>