Only "remaining inside the browser window" to go.
<style>
  body {
    background-color:green;
  }
  div {
    text-align:center;
  }
  img {
    height:23vmin;
  }
</style>
<body>
  <div>
    <img src='cards/back.png'>
    <img src='cards/back.png'>
  </div>
  <div>
    <img src='cards/back.png'>
    <img src='cards/back.png'>
  </div>
</body>

vmin is a unit based on the minimum of the browser window width and height.
Why 23? I found it to be the value that works best for the full game.