We needed to embed an iframe for a third-party form on a client website. The form itself was responsive, in that at a certain width, form fields went from two columns to one column, which meant it wasn't a simple matter of just proportionally resizing the iframe container (the way we do with embedded videos).
Desktop looked fine with an iframe container that had a height of 250px:
But mobile .... didn't:
<div class="js-iframe-container"> </div>