Hey! Public documentation and inspections are a work in progress, so stay tuned.
Editor markup is synchronized between the frontend and backend, which means it can be implemented on either side and will eventually be propagated to the other. When possible, it usually makes sense to implement it on the frontend, because users can see the result immediately in the UI when interacting with foldings, without running into latency-related issues.
However, some plugins still implement it on the backend, either because the computation depends on knowledge available only there, or because the plugin has not been fully refactored yet and some parts still remain on the backend.