I've done this with presentations about web stuff - not even an iframe, just embed the demo straight into the slides. Also, since it's just HTML, you can also

I've done this with presentations about web stuff - not even an iframe, just embed the demo straight into the slides. Also, since it's just HTML, you can also dynamically generate content. I've done things like have a bunch of subslides that just mutate content when I'm trying to telling a technical story, with javascript stepping forward the changes. Works much better than copying and pasting and editing the same slide 25 times. Honestly, I don't think I've made any presentations that didn't use reveal.js (except when forced to) in, what, a decade now? If it wasn't reveal.js back then it was something like it.