There's an option for the basic HTML layout available if you have JS disabled.

That's the second-best approach. The first is using a JS-redirect to the flashy AJAX page, or just overriding all the default handlers necessary with JavaScript (remember, at this point, your content was already (supposed to be) properly rendered for you by the server).

Sure, there is an option for a no-JS version of Gmail, but the default definitely uses Gmail, because the user experience is much better - I think that's pretty clear.

I agree doing some work on the server can also make sense, and that the new pre-rendered JS is an extension of progressive rendering which is not a new technique. It's a new way of doing it, though.

What you've failed to grasp, twice, is that these sites do have javascript-free versions. Which work.

Not that they don't also have JS-infested versions. Which also sometimes work.

In most cases these are separate apps. "Just build it twice!" is necessary in some cases, but is far from idea.

Progressive enhancement is hard, at least for complex interfaces that need to maintain state. Hard enough that many sites, if they support disabling JS at all, do so by writing an entirely new frontend with a simplified feature set.

Ember + Fastboot provides many of the advantages of progressive enhancement, but is far more productive in many cases.