Math.random()

With math-random-polyfill https://github.com/DavidAnson/math-random-polyfill

by Alex

HTML

<script src="https://cdn.rawgit.com/DavidAnson/math-random-polyfill/master/math-random-polyfill.js"></script>

CSS

html, body{
  margin:0px;
  padding:0px;
  background-color: black;
}
div{
  width:5px; height:5px; float:left;
  background-color:white;
}

JavaScript

for(var i = 0; i<20000; i++){
  var newDiv = document.createElement("div"); 
  newDiv.style.opacity = Math.random();
  document.body.appendChild(newDiv);
}