colorsquare.html

<div id='colorsquare'></div>

<script>
  var el = window.colorsquare

  el.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(x * 360)
    var sat = Math.round(y * 100)
    var color = `hsl(${hue}, ${sat}%, 80%)`
    el.style.background = color
  })
</script>