True critique, but progressive loading is not supported by html5 alone. I have been following up with srcset to support proper lazy loading behavior and the day it gets supported you will see it on our site.
I understand the problem and I don't have a good solution -- does srcset really help here?
How about a browser setting where the user can decide whether to load images only when they are seen?
How about it is made the default behavior in any browser, I see no reason why image downloading should ever be not lazy and viewport dependent
its in the pipeline for chrome. should be in testing or dev soon enough.