Yep was going to say Reveal.js too. Earlier today someone posted an integration of Jupyter Notebooks with Reveal.js which looks pretty cool.
https://github.com/datitran/jupyter2slides
The thing that always got me with reveal was nested slides and whether I was supposed to nav with the arrow keys or not. Turns out if you ONLY hit the space bar it will visit the slides in-order. I guess.
But I think the OP was looking for a zoomable surface, which reveal doesn't offer.
slides.com is a front-end for reveal.js, made by the authors of reveal. Nowadays I just write reveal.js directly, but it was nice to have a GUI when I was first getting started