<style>
#ap {
  transform: scale(2,2);
}
</style>
<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <marker id="arrow" viewBox="0 0 10 10" refX="5" refY="5"
        markerWidth="6" markerHeight="6"
        orient="auto-start-reverse">
      <path id="ap" d="M 0 0 L 5 2.5 L 0 5 z" />
    </marker>
  </defs>

  <polyline points="10,30 90,30" fill="none" stroke="black"
   marker-start="url(#arrow)" marker-end="url(#arrow)"  />
</svg>