For a context, I made a game that uses Emscripten and SDL2, the web assembly build works on my local machine. I first tested it directly, and in (local) iframe. It still works. However when I upload the game to itch.io, for some reason the game results in a black screen, further, canvas size inside the iframe gives width=0 height=0. Needless to say, when I directly open the CDN iframe loads from, the game loads and works just fine. Another thing is that the iframe actually works in Firefox, but not in chrome/chromium derivatives. I've made a game with emscripten previously and published it to itch, but have never encountered that problem. 

This issue was magically resolved, after loading the game page, without changing it, everything started working!