<canvas id="canvas" width="500" height="500" style="width: 100px; height: 100px"></canvas>
<script>
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.filter = 'drop-shadow(0px 50px black)';
ctx.fillStyle = '#0f0';
ctx.fillRect(125, 125, 250, 200);
</script>