Its not just about plain text vs. content. Its about doing things at least reasonably efficiently. I commented about gmail taking 500MB on Chrome in another comment. I just checked Mail.app it started with 200MB. I wonder whats the energy budget of all the bloated website and their parsing is on millions of computers. The server side computation allows economy of scale and might have saved cpu cycles for the actual work but I'm sure many times more than that has been wasted by a bloated web.

I don't disagree that web and web applications provide a lot of convenience and I don't want to loose that. But I don't think for a moment that things can't be significantly more efficient.