I\'ve been trying to implement the new srcset approach to responsive images, using the following HTML
I guess you are testing on chrome browser, and the only reason its not working there is because its a chrome feature. See below stack overflow thread (it holds good for latest version of chrome v54 as well):
Google Chrome version 40 srcset attribute problems
Checked your page on IE8 and Firefoxv49 and things work like a charm!
The other thing I observed is that though chrome doesn't download other images, it resizes the one downloaded pretty well. So, in a way we are still in a much better condition had it only been this on page: