<!DOCTYPE html>
<html>
<head>
  <style>
  canvas {
      width: 100px;
      height: 100px;
      background-color: red;
  }
  </style>
</head>
<body>
  <canvas id="canvas1"></canvas>
  <canvas id="canvas2"></canvas>
  <script>
    if (window.testRunner)
      window.testRunner.dumpAsText();

    var iOSPlatform = navigator.userAgent.search(/\biPhone OS\b/) != -1;
    var OSXYosemiteOrOlder = navigator.userAgent.search(/\bMac OS X 10_(8|9|10)(_[0-9]+)?\b/) != -1;
    var MAX_WIDTH = iOSPlatform ? Math.pow(2, 12) : (OSXYosemiteOrOlder ? Math.pow(2, 13) : Math.pow(2, 14));
    var MAX_HEIGHT = MAX_WIDTH;

    function fillCanvas(id, width, height) {
      var canvas = document.getElementById(id);
      canvas.width = width;
      canvas.height = height;

      var ctx = canvas.getContext("2d");

      ctx.fillStyle = "lime";
      ctx.fillRect(0, 0, width, height);
    }
    
    fillCanvas("canvas1", MAX_WIDTH, MAX_HEIGHT);
    fillCanvas("canvas2", MAX_WIDTH + 1, MAX_HEIGHT);
  </script>
</body>
</html>