<feMergeNode>

<svg
  width="200"
  height="200"
  xmlns="http://www.w3.org/2000/svg"
  xmlns:xlink="http://www.w3.org/1999/xlink">
  <filter id="feOffset" x="-40" y="-20" width="100" height="200">
    <feOffset in="SourceGraphic" dx="60" dy="60" />
    <feGaussianBlur in="SourceGraphic" stdDeviation="5" result="blur2" />
    <feMerge>
      <feMergeNode in="blur2" />
      <feMergeNode in="SourceGraphic" />
    </feMerge>
  </filter>

  <rect
    x="40"
    y="40"
    width="100"
    height="100"
    stroke="black"
    fill="green"
    filter="url(#feOffset)" />
  <rect x="40" y="40" width="100" height="100" stroke="black" fill="green" />
</svg>