<!DOCTYPE html>
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1723249">
<style>
#a {
  mask-image: url(#c);
}
</style>
<script>
window.onload = () => {
  c.innerHTML = b.outerHTML
}
</script>
<pre id="a">x</pre>
<svg id="b">
<mask id="c" />
<image requiredExtensions="x">
