In client-side Javascript, document.documentElement.innerText yields only the rendered text, without markup, CSS or JavaScript source.

But the theory was that the source would be different, and only the rendered content would be the same at the sample place, right? How would you find the 'same place' without fully rendering the page?

One way is for the source to use cache control headers so you can just do a quick HEAD instead of a GET on the whole contents.