I never found a way to do this exactly, and strongly suspect it's never going to work with a 3rd party website.
I did however realize that you can scale the contents of an iframe to fit the iframe, which works for me
This seems to be the best stackoverflow on it
How can I scale the content of an iframe?
Here's an example that works in Chrome and Safari (modern) but not sure about Firefox or older browsers. It shows a website embedded in an iframe, and scaled to fit the iframe size.
.wrap { width: 320px; height: 196px; padding: 0; overflow: hidden; float: left;}
.frame { width: 1280px; height: 786px; border: 1px solid black}
.frame {
// zoom: 0.25;
-moz-transform: scale(0.25);
-moz-transform-origin: 0 0;
-o-transform: scale(0.25);
-o-transform-origin: 0 0;
-webkit-transform: scale(0.25);
-webkit-transform-origin: 0 0;
}
<p>before the iframe</p>
<div class="wrap">
<div class="wrap">
<iframe class="frame" src="https://time.is">
</iframe>
</div>