<canvas id="canvas" width="100" height="100"></canvas>
<script>
  var can = document.getElementById('canvas');
  var ctx = can.getContext('2d');

  ctx.shadowOffsetX = 24;
  ctx.shadowOffsetY = 24;
  ctx.shadowColor = 'blue';
  ctx.shadowBlur = 20;

  ctx.rect(0,0,100,50);
  ctx.clip();

  ctx.fillStyle = 'black';
  ctx.fillRect(0,0,40,40);
</script>
