The project is controlled by the community so if a link is not good enough someone can send a PR and replace it with a better one. Also, the ladders should get more rich and detailed over time thanks to the contributions. So I would say that the quality is controlled by setting a minimum level of quality that all PRs must pass and then the quality will improve via very small incremental steps.
Anyway, if the project becomes very active and we manage to have many contributors we could end up creating our own resources but at the moment the fastest way to provide something valuable was to use external resources.
In the future, we might also consider adding a quiz system to check each of the skills but again we would need a significant number of contributors to be able to make that happen.
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