<svg style="display: block; width: 0; height: 0">
  <defs>
    <filter id="crop" primitiveUnits="objectBoundingBox">
      <femerge x="0" y="0" width="50%" height="40%">
        <femergenode in="SourceGraphic"></femergenode>
      </femerge>
    </filter>
  </defs>
</svg>
<script>
var canvas = document.createElement('canvas');
canvas.width = 100;
canvas.height = 100;
var ctx = canvas.getContext('2d');
ctx.filter = 'url(#crop)';
ctx.fillStyle = '#0f0';
ctx.fillRect(0, 0, 90, 90);
document.body.appendChild(canvas);
</script>