In HTML, change text color based on brightness of covered background area?

You can use the following code snippet to change the text color based on the average brightness of the pixels covered by its parent background color.

var rgb = [255, 0, 0];
setInterval(display, 1000);
function display() {
   rgb[0] = Math.round(Math.random() * 255);
   rgb[1] = Math.round(Math.random() * 255);
   rgb[2] = Math.round(Math.random() * 255);
   
   var d = Math.round(((parseInt(rgb[0]) * 299) + (parseInt(rgb[1]) * 587) +
      (parseInt(rgb[2]) * 114)) / 1000);
   // for foregound
   var f = (d> 125) ? 'black' : 'white';
   
  // for background
  var b = 'rgb(' + rgb[0] + ',' + rgb[1] + ',' + rgb[2] + ')';
  $('#box').css('color', f);
  $('#box').css('background-color', b);
}
<scriptsrc = "https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<div id = "box"> Demo</div>

The following is CSS -

#box {
   width: 300px;
  height: 300px;
}

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