[grey-area]

I think simple json documents are much simpler and easier to re-use by other clients in interesting ways than simple html pages. I think the API + client (note, not just traditional web browser) rendering is actually a more "pure" interpretation of what the web can be - data sources and data consumers that interpret and present that data on behalf of their users.

This is an interesting point - if you are representing numeric data like chart datapoints, a representation like json might make it cleaner and more reusable by other services or clients. Of course much data is actually formatted documents or snippets of text, in which case json is not such a good fit and html is perhaps better. In many ways html is a worse is better solution, but that is probably part of its strength - it is very easy to get started with and munge to extract or insert data.

I'm not sure a separate of concerns between server and client is necessary and helpful to all apps, though I'm sure in some cases it is useful (for example serving the same json or xml to a mobile app, a client-side app and some desktop client, or separate teams working on both), but then a server-based solution can easily output both formatted html for traditional clients (web browsers, which are now and in the future ubiquitous), and a json or xml representation for other clients - this sort of separation of concerns between data and presentation is not really exclusive to client-side solutions.