@8b189dd1 Who controls the hosting? If there's a Content-Security-Policy header set it could prevent iframes, though you should see a message about it in the console when opening the page.
@ae43a286 Yeah it's Flickr.com hosting, wherever that is. WordPress VIP. I initially could see the iframe (for including an Internet Archive Book Reader book) in a smushed sort of way, but then after a while it seemed like the iframe was just getting removed before the page loaded. Super strange.