Why doesn't iframe work well?

Question

HTML

Hello. There is a surf site.
Sites are viewed through an iframe, the site itself is on https
On the site browsing page with a timer, I redirected to http so that there were no problems viewing sites without a certificate The problem
is, for some, sites in iframe open normally, and for some, the window is empty, no errors, logs are empty
code:

<frameset onLoad="javascript: frame_footer.startClock();" rows="*,70" style="border: none;">
        <frame name="frame_site" src="<?=$result['url']; ?>" marginwidth="0" marginheight="0" frameborder="0"/>
        <frame name="frame_footer" src="<?=$url;?>" marginwidth="0"
               marginheight="0" scrolling="yes" noresize="noresize" frameborder="0"/>
    </frameset>

Answer 1

Read about headers
https://developer.mozilla.org/ru/docs/Web/HTTP/Hea...
https://developer.mozilla.org/en-US/docs/Web/HTTP/...
And check these headers on the target site, most likely it does not allow itself to be shown in frames