Why some sites does not show in iFrame HTML tag?

The first reason you need to check is to make sure that you are not loading an HTTP website from within an HTTPS website, otherwise
you will get and error. You can see the error by using your browser Developer Tools -> Console.

If you are loading an HTTPS from inside of HTTPS and it is not loading, then take a look at the header X-Frame-Option in response header. If it exists it probably has avalue of either DENY or SAMEORIGIN.

DENY: No one can load the website in iframe. Even the same domain page wont be able to load.

SAMEORIGIN: only a page which is in same domain can load this website in iframe.

#html #iframe #X_Frame_Option