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chromium
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chromium/content/test/data/frame_tree/page_with_iframe_in_multicol.html 

<!DOCTYPE html>
<style>
.multicol {
  columns: 2;
}
</style>
<div class=multicol>
  <iframe src="/cross-site/baz.com/title1.html"></iframe>
  <div>Hello, columns!</div>
</div>