<html>
<head>
<body>
  <img class="custom">
  <img class="big-horizontal" src="large-image.jpg" style="width:500px;">
  <canvas class="big-vertical" style="width:500px;"></canvas>
  <img class="small" src="small-image.gif">
  <img class="data" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABwAAAAcCAYAAAByDd+UAAAAJklEQVRIie3NMREAAAgAoe9fWls4eAzMVM0xoVAoFAqFQqFQ+C9chp4NHvu+4Q4AAAAASUVORK5CYII=">
  <script>
    "use strict";

    window.onload = () => {
      const canvas = document.querySelector("canvas"), ctx = canvas.getContext("2d");
      canvas.width = 1000;
      canvas.height = 2000;
      ctx.fillStyle = "red";
      ctx.fillRect(0, 0, 1000, 2000);

      window.opener.postMessage("ready", "*");
    };
  </script>
</body>
</html>
