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You indicate 2x availability in your HTTP header This part never happens. The client simply signals that it would like 2x images if possible with every request

You indicate 2x availability in your HTTP header This part never happens. The client simply signals that it would like 2x images if possible with every request, and the server sends the 2x images when it is possible, and 1x images when not. This is the exact same thing as has been done with gzip compression for years. The client can choose to get 1x assets by just not sending the header indicating that it wants 2x assets.

tpenzer 14y ago
And what about the performance penalty for the server to determine 2x availability for sites where only a fraction of images are available at 2x res? I'm also not sure I fully understand how we clue the server in on where to look for the 2x version of a 1x asset. In the examples given, do the 2x versions have the same filenames as the 1x versions, but they're in the 'image' directory rather than 'img'? Seems like a significant limitation to force different assets to share the same filename, if that's what's going on here.