The smallest 256x256 single-color PNG file, and where you've seen it (2015)
> Instead of serving a 256x256px image, you can serve a 1px image and tell the browser to scale it up. Of course, if you have to put width= "256px" height= "256px" into your HTML that adds 30 bytes to your HTML!<p>CSS is a thing as well, could just use CSS to force all tiles to the same size, regardless of the image data in them. Something like:<p><pre><code>  .map img {
    width: 256px;
    height: 256px;
  }</code></pre>