<!DOCTYPE html>
<svg width="400">
  <marker id="me" orient="auto" overflow="visible">
    <rect x="-1" y="-0.5" width="1" height="1" fill="green"/>
  </marker>
  <g marker-end="url(#me)" stroke-width="100" stroke="red">
    <!-- Points 2 and 3 coincident -->
    <path d="M50,0Q50,100 50,100"/>
  </g>
  <marker id="ms" orient="auto" overflow="visible">
    <rect x="0" y="-0.5" width="1" height="1" fill="green"/>
  </marker>
  <g marker-start="url(#ms)" stroke-width="100" stroke="red" transform="translate(100 0)">
    <!-- Points 1 and 2 coincident -->
    <path d="M50,0Q50,0 50,100"/>
  </g>
</svg>