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<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <script type="text/javascript">
    document.addEventListener("DOMContentLoaded", function(){
      var canvas = document.getElementById('canvasId');
      var context = canvas.getContext("2d");
      context.fillStyle = "green";

    });
  </script>
  <style type="text/css">
  #canvasId{
    background: black;
  }
  </style>
</head>
<body>
  <canvas width="800" height="600" id="canvasId"></canvas>
</body>
</html>