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Test Info:

<!DOCTYPE html>
<html>
<meta charset="utf-8">
<title>SwapTo OOB access with empty list</title>
<iframe id="ifr"></iframe>
<script>
let ifr = document.getElementById('ifr');
let ifrWin = ifr.contentWindow;
class XFoo extends ifrWin.HTMLElement {
constructor() {
super();
if (ifrWin.didReenter) return;
ifrWin.didReenter = true;
// Clear the active formatting list by closing all tags.
document.write('</b></b></b>');
}
}
ifrWin.customElements.define("x-foo", XFoo, { extends: "b" });
document.write('<b id="b0"><b id="fmt"><b is="x-foo"><p></b>');
</script>