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I'll let the engineers reply directly as I have not worked on it in a while, but just to be clear the renderer used to assess mobileness is not Chrome, its a version of webkit, and as such does *not* have the picture element support yet and it does *not* support srcset yet either.
I created a demo here [developers.google.com...] if you see a kitten that is narrower than it is high, then it is using the src attribute.
I'll ping the team.
[edited by: Robert_Charlton at 1:01 am (utc) on Dec 1, 2016]
https://search.google.com/search-console/mobile-friendlyIt appears that it is the same tool as is available in your GSC account when you use "Fetch as Google" and specify a mobile device.
when it scores the mobile friendliness of a page?