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chromium/third_party/blink/web_tests/editing/inserting/prevent-block-nesting-01.html 

<!doctype html>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../assert_selection.js"></script>
<script>
test(() => assert_selection(
    '<div contenteditable>|<br></div>',
    'insertHTML abc<span><div><br></div></span><div>def</div>',
    '<div contenteditable>abc<div><br></div><div>def|</div></div>'),
    'insertHTML prevents block nesting');
</script>