Further, the iframe is positioned absolutely rather than inline, which means that when I add a line of CSS to the page to scale it up, the text describing the iframe overlays it, rather than scooting out of the way.
To see what I mean, open the Page / HTML Markup dialog, move to the Before section using the picker below the text field, then paste in this code:
Preview in a browser and you’ll see things crashing into one another.
The way around this is to use an inline layout (at least for this part of the page). Draw a box full-width, then set its width to 100% (using the Dimensions part of the Item Inspector). Double-click inside this item, so you see a flashing text cursor, then choose Insert / Action Item / iFrame from the menu. Make the URL, border, and scrolling settings, drag the box out to the width of the parent item, choose a height that works, and with the aforementioned CSS, you should be able to have a flexible iframe. Note that unless the page you inserted into the iframe is also designed in a flexibly-responsive manner, you will potentially have some side-scrolling to deal with when the outer page (containing the iframe) is resized smaller.