chromium/third_party/blink/web_tests/fast/canvas/canvas-filter-global-alpha.html

<canvas id="canvas" width="100" height="100"></canvas>
<script>
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.filter = 'drop-shadow(0px 20px black)';
ctx.globalAlpha = 0.5;
ctx.fillStyle = '#0f0';
ctx.fillRect(25, 25, 50, 20);
</script>