JSFiddle - React, Tailwind, and code Playground

HTML

<div id="alls" class="alls"></div>

JavaScript

var pixs = "";
for(var ia = 1; ia <= 400 * 500; ia++) {
  var r = Math.floor(Math.random() * 257);
  var g = Math.floor(Math.random() * 257);
  var b = Math.floor(Math.random() * 257);
  pixs += "<div class='pix' style='background-color:rgb("+r+","+g+","+b+");'></div>";
}

document.getElementById("alls").innerHTML = pixs;