 Iframe nesting problem

**Describe the bug**

A clear and concise description of what the bug is.

i want get picture  want.png  it works   when html struct in ： <html>   <iframe>  aaaaa</iframe> </html> 
use testSecond.html  
but  it look like not work when html struct in ： <html> <iframe>  <iframe>  aaaaa</iframe></iframe> </html> 
use test.html 




