I think author chose to make too big game for a jam. Could be a good entry but looks like author drowned in scope creep.
To get to the actual gameplay I needed to scale up the browser page, after which I could get to the settings and set game to fullscreen to be able at least start to play. Godot is tricky in screen settings when exporting to web, and needs to be tested a lot on different browsers, systems and screen sizes - it's a tip for the future projects!