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...