java.lang.StringIndexOutOfBoundsException: Index 15 out of bounds for length 15
<html class="reftest-wait"><div contenteditable>something missspelled<br>something elsed#</div>
<script src="/tests/SimpleTest/EventUtils.js"></script>
<script>
document.body.firstChild.focus();
var { maybeOnSpellCheck } = SpecialPowers.ChromeUtils.importESModule(
  "resource://testing-common/AsyncSpellCheckTestHelper.sys.mjs"
);
maybeOnSpellCheck(document.body.firstChild, function() {
  document.documentElement.removeAttribute("class");
});
</script>