<svg xmlns="http://www.w3.org/2000/svg">
  <marker id="m" markerWidth="40" markerHeight="40" refX="10" refY="30">
    <rect x="5" y="15" width="20" height="10" fill="red" />
  </marker>
  <path d="M 50,50 L 100,100" marker-start="url(#m)" stroke="blue"/>

  <rect x="45" y="35" height="10" width="20" fill="green" />
</svg>