The Pym Embedded iFrame presentation type uses Pym.js to embed iframe content responsively, managing both the iframe's width and height. We consider this an advanced topic, and using the Pym Embedded iFrame will require some coding by the end user.
You may find it useful to review the Pym.js documentation.
To use this in Backlight:
To create content for your iframe, download this template HTML document:
This provides basic markup for your HTML page, with the necessary Pym JS script built-in. You may edit the page as you like, or copy-and-paste the script bits from the bottom of the file into your existing project.
I recommend putting these in a /pym folder in the root of your site, i.e.:
Inside the /pym folder, put the index.html and pym.v1.min.js files, downloaded above. When you need to create a new piece of content, duplicate and edit the index.html file, then point the template at it.