This would be great if it evolved in a universal format/spec/feature supported by all major browsers. Only then you'd have to deal with cross web engine compatibilities. And if capsules need to do something non-trivial then you need to consider all the security aspects of having system, file or network level access. Just like any executable. Otherwise with custom app to run capsules it feels like something that can be vibe coded with electron/nw.js rather easily.

But there is something compelling in the idea - there is need for an interactive smart document format that you can share around easily. Kind of like a next gen markdown derivative cross with jupyter notebook where you bundle data, live parametric visualizations, code, audio, video, asciicinema like playback, comments, metadata, file artifacts etc