How do I hide all page elements in an iframe except one?

Hi! I tried to figure out how to use :not() with CSS to hide all things expect one part of an iframe. But after almost one hour its still not working.

Does anyone know how to do this in CSS?

And is there a way to apply this css only to the iframe in Construct 3?

I want to hide everything except this part on the iframe:

<section id="site">

and on another iframe:

<div class="detail-tabs">

Does this work with an external page?