Paypal Standard not working in embeded civi form
Yes, it's a security measure of paypal, you can't use it in an iframe. 

I would suggest that instead of embedding the donation form, you simply put a link to your civicrm contribution page.

You might have another payment processor that is willing to work from within a frame (try stripe if you want), but you might get blocked in the future, because trying to "hide" what is the real domain of the form asking for money is something scammers would do when trying to trick victims to send them money when they think they are paying someone else