This doesn't address the asymptotic impossibility of maintaining true 100% uptime, but... I think you can maintain uptime even when the servers aren't availabl

This doesn't address the asymptotic impossibility of maintaining true 100% uptime, but... I think you can maintain uptime even when the servers aren't available with something like Meteor (a single-page web app built with Javascript...doesn't have to be Meteor, but Meteor should be able to handle it). It's not easy, but with Meteor's events indicating availability of the server should allow you to cache pending changes (when the server is unavailable) and send cached changes when the server becomes available again. Meteor gives you full control over a client-side session, such that as long as the user doesn't reload the page, the user can continue using the app. There are hooks to detect when the server reconnects, so you should be able to write some custom code that updates the server with any pending changes that occurred when the server is unavailable. Reloading the page would kill the pending updates, so you'd need to have the users use some sort of kiosk-like browser that prevents reloading the page and navigating to other pages. There are other issues to be considered (what if the pending changes are invalidated by another user's changes?) that are very challenging, but can be handled depending on the user's other requirements.