Characters ending XML Processing Instructions (WIP)

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<!-- This vector causes Firefox to hang please fix-->
<!--
<iframe src='data:application/xml,<?xml version="1.0" encoding="UTF-8"$[chr]><x:script xmlns:x="http://www.w3.org/1999/xhtml">window.parent.postMessage("$[chr]","*")</x:script>'></iframe>
<script>
  window.addEventListener('message', e => console.log(e.data));
</script>
-->