<svg style="display: block; width: 0; height: 0">
  <defs>
    <filter id="merge">
      <femerge>
        <femergenode in="StrokePaint"></femergenode>
        <femergenode in="SourceGraphic"></femergenode>
      </femerge>
    </filter>
  </defs>
</svg>
<canvas id="canvas" width="100" height="100"></canvas>
<script>
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.filter = 'url(#merge)';
ctx.fillStyle = '#00f';
ctx.strokeStyle = '#0f0';
ctx.fillRect(40, 40, 20, 20);
</script>