<!doctype html>
<title>Marker with mask reference cycle</title>
<svg>
  <mask id='mask'>
    <path d='M0,0h50v50h-50z' marker-start='url(#marker)'/>
  </mask>
  <marker id='marker' mask='url(#mask)'></marker>
  <path d='M0,0h100v100h-100z' fill='green' marker-start='url(#marker)'/>
</svg>
<script>
  document.body.offsetTop;
  document.querySelector('#marker').setAttribute('fill', 'url(#nonexisting)');
</script>
