If you are using a browser that does not support the srcset attribute, the default image will be loaded, and no magic will happen.
Can you please tell me what your device, browser and window width are.
Chrome in a large screen.
You must reload the page, it doesn't change at runtime.
Actually, it does. If the image size increases, a higher quality version will be loaded if available.
Then I have to assume that I have the same problem of
edpichler (Firefox 40, Ubuntu) (or I don't understand the purpose of the demo)
It seems to work - for me, in Chrome 44 on a Mac, at least - if you start with a mobile-width browser and then resize the window wider. Resizing from large to small doesn't seem to work.