I have a similar requirement, and I also wondered.
Unfortunately, iCloud data does not seem to be available for the web application. And if you found to hack it, it would still be an unreliable decision, since Apple could always make changes along the way, which intentionally violates your web component.
Change With CloudKit, this is no longer the case. It comes with a Javascript API for the explicit purpose of providing a web version of your application.
Clafou
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