<!DOCTYPE html>
<body>
  <canvas id="canvas" width="100" height="100"></canvas>
</body>
<script>
  var canvas = document.getElementById('canvas');
  var ctx = canvas.getContext('2d');
  ctx.fillStyle = 'rgb(200, 0, 0)';
  ctx.fillRect(0, 0, 100, 100);
</script>