<!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>