I read this article here that talks about progressive enhancement for javascript and the author mentioned:
First, build an old-fashioned website that uses hyperlinks and forms to pass information to the server. The server returns whole new pages with each request.
Now, use JavaScript to intercept those links and form submissions and pass the information via XMLHttpRequest instead. You can then select which parts of the page need to be updated instead of updating the whole page.
I'm a little curious if does that means returning html markups at the server side instead of json, which usually means building the markup on the client side? Is there a disadvantage for this approach?
Also, I notice applications, for instance Facebook, looks pretty crippled when I disabled Javascript (can't post updates etc.) Does that means that it does not handle graceful degradation properly?
Does progressive enhancement means no json with ajax?
No, it most certainly does not mean that. If JavaScript is disabled, there is no XMLHttpRequest, so there is no ajax.
Now, use JavaScript to intercept those links and form submissions and pass the information via XMLHttpRequest instead.
The JavaScript bits that intercept links and form submissions can freely change where the requests are made, URL parameters, and so on, which means that ajaxified URLs don't have to be identical to JavaScript-less ones. For example:
<a href="/some/page.html">linky</a>
could be intercepted and turned into an XMLHttpRequest which is actually made to
/some/page.json, or/some/page.html?ajax=1, or/bibbidi/bobbidi/boo (for all that it matters)