Or simply flushing out the <head> of a html document before the body is generated so the browser can start pre-fetching resources. That's the main use case that motivated Rails to implement streaming.
I've been using hogan.js to compile mustache templates to JavaScript functions on the server-side and write them to a js file that is then served to the browser:
html and css source is not execution. Getting distribution, partnerships, sales, etc. right is execution, especially since they don't clone tech-intensive companies (or at least not the tech-intensive parts)