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chromium/third_party/blink/renderer/core/testing/data/inner_text_test1.html 

<html>
  <body>
    A<iframe src="subframe-a.html"></iframe>B
    C<iframe src="subframe-b.html"></iframe>D
    E<iframe src="subframe-d.html" id="d"></iframe>F
  </body>
</html>