I was following the MDN example, which explicitly set the sizes, but it seems 100vw is sufficient.
According to this SO post:
https://stackoverflow.com/questions/28650327/responsive-images-srcset-not-working
it seems like browser cache hides the results. Testing FF Developer in Incognito showed the correct images loading in Network tab.
Thanks for your help, and hope this info is useful for others. It may be worth adding to the docs!
