In the blink rendering engine, partial rendering of a progressive jpeg is pretty much the lowest priority task for CPU time. That means unless you have a very slow network and very fast CPU, you will never see half rendered progressive jpegs.

That's the right design decision, because rendering a progressive jpeg repeatedly is rather expensive - as well as redecoding the whole jpeg, you also need to re-render any text or effects overlaying the jpeg, and recomposite the frame. And then you're gonna have to do all that work again when a few more bytes of progressive jpeg arrive from the network...

I expect they do. It might just be that people aren't often encoding JPEGs such that they can take advantage of the support (or that your link is generally fast enough that you don't see an intermediate render of the data).

Yes.