Can an iframe be used in Plone 5 ?
It seems to me that code gets stripped out, even if I disable the filtering or putting them in a (raw) html file or a page template.
(the iframe is there, but inside it, only empty
Have you possibly mixed http and https?
It looks like one needs to reload the page (more than) TWICE to see the content. (Save - Reload - Reload)
It's a little wonky. Those transformed values are cached. So if you add an iframe, then change the filtering setting, it'll still show the value stripped. However, if you edit, save the page again after you've changed the setting, you'll get the unfiltered version.