This is an interesting take on documentation - mainly because I fail to see the value proposition in paying for the functionality provided.
Speaking from my own experience:
- Notifications. I am not sure that I've ever needed to know when a doc is updated, because if there is anything radical coming on the market (or in a spec proposal), there are other avenues to find out about it.
- Collections. That is already a functionality in the browser that is not locked into just one documentation site.
- Offline mode. There is Zeal[0] if you like client-side software and devdocs.io[1] if you like browser mode.
Combine all that with the fact that it's just for MDN, and the appeal kind of disappears. YMMV, of course.
[1]: https://devdocs.io/
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